3 phase heating control of hot water
But i also must now get the over temp alarm into the circuit to switch the contactor off as well.

I think you can see the problem with this. If the contactor contacts have welded then it doesn't matter what protection you add to the coil - the contacts will remain welded.
The standard course of action is to use the over-temperature trip and a separate contactor with its contacts wired in series with the normal one. The safety contactor will see very few cycles in normal operation so the contacts are unlikely to weld.