Have you checked for water flowing adequately into and out of the heater matrix, that would be my first check, this should also double check that the water pump is pumping the water round properly.

I suspect the overheating may be cyl head gasket, your mechanic should be able to do a sniff test of the coolant before going delving.

Has your mechanic actually checked the temperature, the gauge may show hot but that may not be a true reading and its only coming up to warm when in the red.

A leaking head gasket won't of itself cause an engine to overheat, what does is the pressure in the system expelling the coolant so if you are not loosing coolant the problem might lay elsewhere.

You say you have had a new water pump so its unlikely although not impossible for it to be that again. You may possibly have an airlock, disconnect one of the heater hoses and pour water into the filler until it comes out of both ends.

Classic PD head gasket.

The OP claims to have replaced the water pump.

I'd go for a blocked radiator matrix which any semi competent garge could find.

So change garage first.

Test water flows

Sniff test coolant

pressure test radiator cap.

hi thanks to everyone that has replied.. i have already done the water pump and this was not the problem.. i have since flushed the system with holts 2 step and a hose pipe, still have the problem. As i am in italy working i took the car to the mechanic work use he looked at the car for 15 mins doing no tests and said he had this problem once before and suggested it was the HG and would be €2000 to fix.. The car didn't cost that and i dont see how even after driving 1000 miles to Italy at 45mph there is no water in oil or oil in coolant. When i got the car it worked fine but the heaters never have and it climatronic too.. If i do not travel anywhere and sit the RPM at 2-3000 for upto a hour it is fine and heaters blow warm before you set off anywhere. Soon as you go anywhere the heaters blow cold and then going over 50 it overheats. i did notice after using the car NOT going over 50 that there was pressure coming from the expansion tank. I checked the cap which was on full so i took it to the car park and boosted upto 50-60 a few time till the temp went up and then got out to check for pressure noise again, but noticed before even removing the cap coolant had been released from the overflow. The coolant was not over filled and the cap was fully on, but i have checked and there is meant to be a metal rim where the rubber sits from the cap. On mine there isnt its just a ruff rusty mark, could this be the problem?? If not can anyone help as i dont trust the garages in Italy and dont return for a while and want it sorted. Also seen a HG tester on ebay for 35.00 do you recommend it..

You do not always get water in the oil or oil in the water it depends were and how the HG has blown.