Heat Problem..... lack of

If you have not spotted any leaks, first call would be to remove the oil dipstick and see if the oil is over the 'MAX LIMIT' or 'milky' (this would indicate a head gasket failure)

thats in the normal range and not milky looking.

A cracked reserve tank? This can spray out coolant when under pressure even from a tiny crack. Mine went at 9 years.

as i said no signs of leak

perhaps this stuff you added has blocked the heater and main radiator ? it wouldnt be the thermostat, because if sticking shut the engine would overheat and give you a great heater , if stuck open a poor heater and the fan wouldnt run , Whats the temp gauge doing? stalling is most likely the throttle flap and body needs cleaning of carbon deposits ...

well the car is now overheating (thermo gone to redline) and therefore just broke down :-/ what you recon is causing the car to overheat now :(

lack of water or thermostat stuck shut .. the heater wont work with a lack of water so maybe thats the problem ..

Also doulbe check that expansion/header tank. Suffers from hairline cracks down low because where it's housed gets real hot. Pull it out and look closely where the pipe attaches to the bottom.

I mentioned that but he insisted it has no leaks