OP here

Well, the bolt would only turn a quarter turn, I flooded it with oil and worked it for half an hour but no joy so with the help of my neighbour and a metre long bit of scaffolding pole attached to the spanner I eventually got the bolt out.

Every turn of the bolt took a huge amount of effort even with the scaffolding pole, the thread is knackered. Only question remaining is what the thread in the nut that's welded to the mechanism is like - probably knackered too!

I'll have to see if Witter will sell me a new mechanism