Hopefully I can help..
Can you tell me if possible exactly what parts / testing / checks has been done upto now on the car to try and resolve this problem?
Is the car actually loosing any coolant?
Any signs of excess smoke from the exhaust?
Do you hear the main fan kicking in when this overheating occurs?
On checking the coolant several times, there is no loss - also had friends to help me check and all felt the same.
No signs of excess smoke from the exhaust.
No, I do not recall hearing the main fan kicking in when overheating occurs.
Please note the car is not from 1956 - it was new and registered in Sept 2006.
Parts/testing/checks upto now:
A/C not working since 2/10/ 2010.
18/10/2010 Water pump and cambelt changed, water pump repair kit, bolt, v-belt, drive belts
30/7/2010 no faults experienced but coolant temp sensor replaced.
27/5/2010 Cooling system checked, pressure tested, thermostat and cooling fan operation all OK and not faults present.
26/3/2010 Thermostat seal and housing replaced.
28/9/2011 Water header tank fitted.
7/6/2012 New clutch kit and front brake pads.
Hope this helps - I have kept all my paper records in date order and have provided this information from all my file paperwork.
I will print your suggestions off and ask the garage to check on all your suggestions - don't worry, I will only print off the advice bit and not your details and I really keep my fingers crossed that the car can be sorted out as soon as possible and that your suggestions will lead to a solution of the problem as it is so annoying.
like I say before if they have infrared thermometer that would confirm whether the engine is actually overheating and the gauge isn't giving a false reading..
If you need to ask anything else then just continue..