Or head gasket failing and pressurising the water system.
a distinct possibility...
however before you get to that point I would check that the coolant system is getting to pressure, if there is a leak anywhere then it won't be and the boiling point of the coolant will be lower.
step one fit new rad cap, its amazing how often this is overlooked when looking for cooling system faults..if its failing then the system would not pressurise properly and coolant would boil at a lower temp, check hoses and jubilee clips as well, make sure its nothing ''obvious''.
step two google coolant system pressure tester, you can make one cheaply for a tenner,pressurise the system to about 25 psi with the engine off.
also worht adding a dye to the water thats fluorescent under blacklight..kit from ebay £10.
listen for any hissing or bubbling,actually better if you find a problem at this point.
step 3 compression test, if you are anywhere near guildford I have a mate with a compressor and a tester, if not any mechanic worth his salt should have one or access to one.
after all of this if there is no leak the after releasing the pressure start the engine and let idle for a bit..5-10 mins, switch car off and listen for leaks..if one is there then the system is very very likely being pressurised by a head gasket breach.
at the outside it could be a failing water pump or thermostat..last two culprits are unlikely though as they either work or don't.
with a jts if the pump was failing the cambelt would be next in line to go..
stat would be obvious as the car would either be running cooler if failed open or running hotter if failed shut.
I hope its something simple as a HG is a pig of a job!