May I thank all on here who regularly suggest the use of an angle grinder for every conceivable problem and a few inconceivable ones too.
I had a broken rack bolt in a wooden window, why do these things always fail locked? After a fruitless poke around with a screwdriver I began to wonder what the advice on here would be. I'm sure that "use an angle grinder" would have been high on the list. Any way there was a slight gap between the window and the frame that I could see the bolt through. I removed the guard from, fool I know, and fitted a very thin disc to my angle grinder. Shortly afterwards the bolt was cut in two and the window opened.
So thanks to all of you.