Have you tried contacting BSA? Can you post any detailed clear pics of your gun?
Airsporters can be rather random, if you've a good one, it may be worth spending a few quid on but the quality control BSA had through the 's and early 90's was crap!
The main problem of the Airsporter was the alinement of the loading tap with the barrel. Try fireing a pellet into something so that you can see exactly what it looks like. If you can see the rifle marks perfectly all around the pellet after it's been fired, then it's ok.
If, however, it looks as if part of it's been shaved off, it's a duffer and