Broken bolt

I tested the water pump yesterday morning and it was found to be pumping ok in both directions. The only remaining issue is now the removal of the broken bolt, so I can replace the thermostat housing. I had no success with the mole grips yesterday so the next step is to flatten off the top of the bolt and punch it and drill a pilot hole. I will then try and use an extraction tool. A week with wd40 oil may also help. The only issue is the the small width of the bolt. I have also bought some gasket sheet for the one that broke on the housing, hopefully will be able to replace soon.