Overheating MGF after Engine replacement

Hi all, I've just joined and was hoping for some help.

I have a 1998 Mg MGF VVC 143bhp , the car has just underwent a decent amount of work.

* New engine
* New water pump 
* New thermostat
* New and upgraded radiator and fans 
* Bled a series of times from all three bleed points. Jiggle valves also appear to be free. but he was only able to blow test 1.

The car is back up and running, it idles fine but after 15 mins of driving it begins to over heat. 
I'm Currently in Northern Ireland and i was wondering if there's any MG mechanics still around as my mechanic cant figure out 'wrong. 

I've been told to replace the cover cap on the coolant reservoir incase it is leaking but it appears to be holding the pressure due to the build when it is released. My mechanic has said the coolant is flash boiling when its reaching the engine from the radiator.

Any ideas of who to call or where to go, Im not super mechanically inclined but I'm hopefully getting it back this week and if there any ideas for any things i could check whilst i wait that would be great. 

Some of the thermostats for the K series on general sale are known to open at too high a temperature. If you haven't done so already try removing the thermostat or refit the old one after gutting it so it is non restrictive. See if this helps with the overheating

I have a coreless thermostat you can have if you want, also in Ireland so DM me and i'll post it tomorrow.

When I first had my F and worked on it over a period of time I decided to replace the 88 degree stat with a 82 that include a jiggle valve, I believe it might have been the same as fitted to the Metro's not 100% sure now.
It seems to be working ok after a few years now, but the only possible down side of going with a lower temp stat is that the engine may not reach optimum temp for performance. I am now working (very slowly) on my TF 160 VVC which I had thought about going with the PRT system but decided against it for a number of reasons, anyway I fitted the correct 88 degree temp stat in it which also has a jiggle valve, I had to reduce the circumference to make it fit as it was for a Land Rover Discovery. Not tested yet because as I stated, I'm working very slowly on this project. The car was kindly donated to me by Mr Alan Ritchie to whom I am very grateful!
Good luck with sorting the issue. Glyn.

Hi so how are things going with this after the advice given by different sources?
Glyn.