With much larger threads (Triumph motorbike clutch hub) I used plastic padding glopped onto a piece of perforated zinc. A bit of oil on the threads makes a good release agent and the bodge shrinks fractionally when it sets as well. In this case, I might use the same technique, but stick a small screwdriver into the bodge until it had set, then unscrew it.
Obviously, Michael needs to get an idea of the OD of the thread,
Being slightly mischevous I thought I would hand you this link to one of Tony Griffiths' web pages
discovered it the other day and was amazed how many different thread types have been invented over the years.
Fortunately it normally boils down to a small subset, with age and country of origin being good clues, as are type of material it is going into, and application (e.g. BA used to be popular for electrical fittings). Of course if its part of a larger bit of kit, then find out what the rest of the threads are (especially if the problem one is mangled).
If you find the ID of the hole then you have an estimate of the tapping drill size, then you can look through a link like this one
to identify the most likely candidates. You can then elminiate by trying known threaded screws in the hole, and see what is left.