How long to wait - coolant bleed

Delbourt - I've been there many times.  Present thing included!  I advice simply walking away for a few days. Forget it.  Then - perhaps you could consider starting again?  Drain it - see how much is in there.
Jack the back up, open the bleed valves and fill again with plain water (considerably cheaper).  See how that goes.   Good Luck !!
  

Good advice to “walk-away” for a day or two.
Briefly. I know the heating and cooling worked before I drained it back in January.
The coolant that came out was clear with good colour.
I have taken the hose off the “T” above the expansion tank and blown down it with the associated hoses clamped. I can then blow and suck on that hose and sense some movement.
if I release a clamp water will flow out of one of the other legs.
I think that implies that the 8mm restrictor is clear although the jiggle valve might be inconclusive (and not something I’m going to investigate further as that means dismantling a lot of “stuff” bolted to the cylinder head.
This car would have left the production line with the PRT and with low mileage (and its history) not much opportunity to be messed with.
Of course anything is possible.

By the way the rad bleed screw is 12mm x 1,25 pitch

My car is a fairly early car, it’s useful to note the thread form is, as reported earlier, the same, at M12 x 1.25.
A contributor mentioned that a spark plug would make a temporary replacement; it will not. The thread diameter, certainly for our cars, is M14. I have no doubt there will be plugs of M12 thread but unless you have an engine to take that size, you are unlikely to have one about.
Just why the coolant circuit on some cars are so difficult to bleed is mysterious. My car is a Mk1 f & from the first circuit flush, it has been straight forward. Subsequently I have removed the original thermostat & fitted a PRT, refilled it & after a good burp from the bleed points, it has been fine.
This has been consistent for water pump changes, longitudinal pipe replacement plus any number of farty about jobs.
I’ll be draining down next week to change a perished pipe, I don’t expect any problems.
I wish you the best of luck but as you can see I really don’t have any useful comments to add.
M

So this was make my  mind up time.
After grovelling under the front the two hoses to the rad could be felt. With the engine running these got too hot to handle for more than a second or two. But squeezing them made no discernible difference as car as I could tell.
With the engine running the level in the tank would threaten to overflow before retreating to its initial level.
After observing this for several cycles the engine speed was increased - probably over 2000 rpm and with the bleed hose from the rad open water could be seen to flow through the hose into the tank containing minuscule air bubbles that reduced in number. At that point the the throttle was released back to idle. Instantly the tank overflowed with very aerated coolant (and a lot of it).

After much cursing and a good lunch it was reluctantly decided to drain it all down and start again. With the front raised up by 4” the underfloor hoses were released at the rear and coolant drained to a bucket. That volume equated to as near as anything 7 Litres. That obviously does not include what was lost before lunch (and that was a lot - it went everywhere- and residues in mopping up rags. But it suggests a significant air lock.
The jiggle valve was blown through and the 8mm restrictor checked for blockage. As expected both were free.

So now with the hoses re connected, the front back on the ground and the back end off the ground by 4” I’d like an opinion on the following please.
When it comes to refilling should that be done slowly with obvious breaks in the process or quickly so that the head tank never runs empty?
With or without the bleed valve on the radiator open whilst filling?
Im sure at least one or more of you will have done this successfully with a PRT system. Please comment.

You are getting somewhere, very well done.
I would fill it slowly with all the airing points open. If you do it slowly the water will find its own level and rise slowly but surely.
Close each airing point as and when you see water. You may have to revisit them later.
Good luck...this is it...no selling of the car needed.

As with Brian - 5 litre containers -  pour it steadily in.  Obviously with heater on hot.   Total should be about 12.5 litres. 
I cheated in a way as I have an MS Rad filler /  bleed.  SO I fitted short lengths of plastic pipe to that and heater bleed 
that were slightly higher than the expansion and would be easily visible if water / coolant  emerged.   (Would not pour over the floor) 
+ stick a hypo in there for a suck if necessary.   Had no trouble.

Here are some interesting thoughts!
On previous attempts I had hoses fitted to hose spigots fitted to rad and the engine bay manifold bleed points and flow/seal control arranged with small G clamps.
As the engine bay one was opened water would tend to be drawn in from the hose column. My theory is that the 8mm restrictor configuration operated like an “ejector pump” so that as engine speed was increased the velocity through the restrictor increased and the ejector concept came into play. With engine off and head tank full water would rise up the hose through the spigot.

Why water would not flow through the rad bleed with engine at idle is bit of a puzzle. At rest it would come to the top of the rad and some would flow out. With  Engine running at speed and faster impeller radial tip velocity the pressure would be higher. So flow would be initiated. Why nothing at idle?
The next question is why would releasing the throttle give rise to expansion of the coolant (presumably the entrapped air) but with a stable temperature why sudden release. Presumably a sudden release of pressure enables decrease in the vapour pressure and subsequent expansion. But why wouldn’t that air come out of the system?

Off the top of my head..if water came to the top of the rad you had flow down one pipe pushing the water out...but you didn't have a return path and hence no circulation when the rad point was closed.
I wouldn't fit any hoses etc. I would open the airing points and fill it relatively slowly.
Good luck.

Sorry if I'm being a bit dim - but you are not attempting to do it with the engine running?

https://www.the-t-bar.com/forum/22-cjjs-guides/401-coolant-system-how-to-fill-and-bleed
Jack the back up, open all bleeds and proceed steadily.
Smooth growbags are useful for catching the water /  coolant  dripping everywhere.
Also jolly useful as Birdie Bath.
 

Thank you for the thoughts.
The “how to” suggests filling with vent points closed not open which is different to recent suggestions. 
Regarding the thought about the “return path”. With the PRT arrangement the return is as far as I can see on the near side of the car and the hose connection on that side is at the bottom of the radiator. So whilst an interesting thought it I don’t think it applies in this example. I’d need to go and re check the configuration to satisfy myself on that.

Vent points were both open and closed alternatively when running. The system was initially filled with the engine off.
See this link for an example of the ejector pump https://www.transvac.co.uk/learning-centre/how-an-ejector-works/
And I think that explains the observations associated with the bleed point in the engine bay.

A few “sums” and guessing pipe and length dimensions associated with the heater arrangement I think the air “hold up” could well be in that heater system. And that potential volume also accounts for some of the differences between what I collected yesterday and a full charge. Getting that to fully drain without jacking the front up by much more than I can achieve is not possible (think where the hoses splice into the main system).

All interesting stuff but a great big pain in the you know where.

Here is my reasoning behind leaving the vents open and filling slowly.
When you fill a c/h system you have a header tank in the loft or a sealed system with mains pressure.
Both create a head of water that can build up and force air out of the rads as you open them.

Our cars have almost zero static head of water..maybe 6-12 inches so there is no real head of water to push air out.
With the vents open the water will slowly rise and the air in front of it can escspe rather than form a vaccum.
Either way..todau is the day you fill your car.
Good luck.

Increase the water head by using a crap header tank cap, cut out the top & attach eighteen inch of plastic plumbing to it; that would give you a head. Create similar for your bleed points using translucent pipe & with everything vertical, you have created a (complex) manometer.
Fill the revised header tank & see where the fluid rises to, all should be the same. Use Vince’s syringe to draw fluid from test point.
Let it stand & monitor levels. Lock each section off in turn & you should, providing the system is sound elsewhere, - - - - You join the dots.

M