possible thermostat fault??

Get your garden hose out.

Remove both heater hoses. Put your garden hose on one, turn it on full, and see if water shoots out of the other, as the heater may be blocked with debris, especially if the engine has been run without antifreeze, or worse, if someone put magic miracle head gasket repair in the water.
Some of these contain waterglass, as in what we used to grow a chemical garden with and turn antifreeze to jelly.

Try swapping the hose to the other outlet to blow it clear, then try careful filling & bleeding.

Chemical cleaners, or a big dollop of hydrochloric acid (muriatic acid) and run the engine to hot, are a last resort, these tend to remove the scale blocking pinholes in the radiator and heater, and set them off leaking.

might sound silly but whats the best way to re fill the cooling system without getting air locks, or to try and prevent them?? do i have to have the heater controls in a certain position aswell?

thanks

usually have the heaters on and set to HOT. Usually fill and with engine running and cap off lets air out but also use bleed valve to let air out if there is one.

all done flushed the heater matrix out , re filled and all is good now...............its too hot!!!!!!!!!!!