Well, after yesterday's fun, I knew today was going to be just as eventful. Cooked brekky, cuppa, and off I went.
Fortunately there was enough of the broken thread to get a hold on. And with a set of thread extractors, drill, gas torch, the first one fell out after a few hours twisting and banging.
The second one snapped in half again, so I ended up having to drill all the way through. Luckily that one wasn't an enclosed thread, so it soon came out once I'd broken through to the other side.
With both threads clear, it was a simple enough job retapping and then finding a couple of bolts in Ian's huge store of bits n bobs. One of the great things about coming home and working on the car is having access to just about everything you need, without having to pop down Halfords for it (17mm sockets for example ).
Job done, time to hit the road and get home. Fingers crossed it's sorted and ain't going to heat up no more.