Jeebowhite, extensive research thus far suggests that a new igniton barrel will cure your problem, something to do with the microswitch on the steering lock not being depressed properly when removing the key.
However, once it is off it will not come back on if that is the problem.
I think Stoney may have correctly guessed the source of my problem - the fuse box under the bonnet. Here is a photo of it when I opened it up (I removed the ECU fuse to have a look under it - mild corrosion on the bottom most pin).
I'm guessing here, but I believe the last person who took the lid off didn't refit it correctly, and it has filled with dirt. Is it likely this is the cause?
If so, how difficult is it to remove the box and give it a proper clean? (only the photographed area is affected)