Coolant system issues

You want more ideas ? Just follow a logical progression starting with the quickest:

If you have replaced the cap and there is no improvement then start disconnecting all the cooling system hoses front and rear and give them plus the underfloor pipes and the rear engine pipes, a serious blast through with the garden hosepipe. Flush both ways.

Check both jiggle valves are clear and not gummed up.

Do a test for exhaust gasses in the coolant at this point, they are readily obtainable on Ebay and easy to use. If the fluid changes colour you've got a more serious problem. But if it stays blue then proceed with the following :

Remove the thermostat from it's housing to check it is opening correctly by lowering it into a jar of boiling water. Replace the thermostat if in any doubt. It's often best to replace the O rings when doing this as they can stretch with age which makes refitting the thermostat housing an unecessary struggle.

If you havent already removed and flushed the radiator then thats worth a try now as they can be partially blocked by sludge.
Use a cooling system flushing solution such as Wynns or STP.

If the problem still persists and theres no indication from service records when the waterpump was last changed then that has to be next in case the impeller may be slipping under load. Whilst it is out get the hosepipe into the waterpump housing and blast any crud out of the waterways in the block. Then put the hosepipe in the outlet on the other side of the engine and flush it back that way before fitting the new waterpump.

Very detailed and real world information.
Sometimes we just have to get stuck in a little deeper.
Good luck..lot's of good advice there.

Thanks for the advice. I'll give this ago then update when I have news.

I didn't want to sidetrack other threads so I'll jump in on this one. 
As David advised I was going to bridge the fan relay but it would appear that someone else has already beaten me to it. 
this can't be a factory fitting, can it?
I also checked the 15 amp fuse and that's fine. 
I have ordered a new coolant cap and will break out the multi meter and get back for more advice when I have more information. 

I’ve seen these before, they were made by Rist Wire & Cable as a test plug, I believe. Useful & safe for testing a relay system & once fitted with the appropriate fuse practically, practically idiot proof.
Practically.
M

Can someone help me on the diameter of the radiator hose ?
Is it 40 mm internaly ?

Smaller than that i would think.

Measuring the inlet/ outlet on a spare radiator shows around 32mm, so i'd expect the internal hose diameter to be somewhere close to this.

I think that the link shown in my previous photo is a standard fitment on a car with air-conditioning, it has a different relay pack just off to the side.
I have been looking through various topics and wiring diagrams but I have no idea what I'm doing. 
 

I followed the advice and replaced the cap, flushed everything thoroughly and re bled the system several times just to make sure. 
I'm glad to report that I haven't had any further problems with the coolant side of things 🙃 
I still have a few electrical issues so I have ordered a manual because I'm not the greatest with this stuff and trying to cross reference on a small screen is a bit of a nightmare 🤔 
Thanks for the great advice here again. 👍 

What are your electrical problems, between Roverlike & myself we might be able to shine a bit of a light
M

Hi, i recently bought a 1996 mgf, the temperature gauge isn't working I've changed both temperature sensors (brown and blue) and tested the gauge with wire and it shot up.  I'm thinking a possible plug/connector issue. Any idea where i can get a replacement? Any help would be greatly appreciated.

G'day CallumE  and welcome to The-T-Bar forum. Whilst not related to your current enquiry, may I suggest you check out our check list for new members and owners:  https://www.the-t-bar.com/forum/54-how-tos/99326-a-check-list-for-new-owners