Managed to get a couple of hours in the garage/workshop/playroom on Saturday afternoon. Ended up drilling out the snapped stud and fitting a Helicoil. I hate using Easyout's, they can be good when the snapped stud is loose already, (who snaps a loose bolt/screw?) but if the thing is seized or corroded in place, I find that the simple act of using one, forces the remaining stud material outwards onto the thread and nothing happens until the easyout snaps - then you've got two problems!
Anyway, coolant Foo Foo valve sorted and back in position.