Nissan Primera P11 GT Overheating Issues

I've just swapped the thermostat for a fresh one. Just giving the sealant time to cure before refilling the coolant. If it's still not sorted though I'll definitely try bypassing the heater rad. Is that just in case it's blocked or leaking inside the car? 

Had a bit of a heating issue with the wife's Almera last year, replaced Thermostat and must have been a dodgy one as it didn't solve it, Removed it and ran without one for a few weeks and solved if not reversed the problem. Ran too cool on Motorways then lol

I fitted the pump and stat first, took the pump out again when I did the timing belt to have a nosy and all looks fine, bearing on its still solid and belt was replaced too and is the right amount of tight.
Pokey bit of the thermostat goes into the engine side right

Hey guys. Sorry for being terrible at replying. Decided to throw stuff at it and see what sticks. With doing the wrench work myself cost wasn't too bad.
So it's had head gasket, timing chain and a new rad fitted. All seemed well until tonight, it's been on several hour to 2 hour runs at high speed and sat in traffic too. Tonight a tootle down to the local asda had it overheating again, boiling coolant spewing out the rad cap and generally not happy. That's on a half hour steady drive!
Feeling pretty down and really just not sure what to do next!

Temp OK with the heater on full but overheating with it shut off can be a sign of a radiator problem.