Disaster, there is X4 8mm bolts holding thermostat on to engine, i got 3 out no problem, the last one the socket is just slipping as the head of bolt is rounded, I managed to hammer a 7mm socket on to it, but that just slips of aswell. There is no way to get a spanner on, and I have tried a 6 sided socket but that just slips aswell.

Any bright ideas how i will get it out.

mole grips

Can not get mole grips any where near bolt

its mole grips or nothing i can tthink of anything that will fit in the small gap you may have to dismantle some of the pipework etc to get into it mate