Thanks for your thoughts.
It's frustrating that the law isn't clear on the definitions of a air pistol / rifle.
Not sure how the authorities would ever know that it was once 60cm in length and my reason for fitting a folding stock would be for transit purposes and pest control in confined spaces. Not sure that would cut it in court though, nor do I ever want to be in that situation.
My thinking was that even though airguns are excluded from the 30/60 rule, it nevertheless meets the 60cm total length currently with a fixed stock so therefore is legal (although the screws are not AT which must be a new thing?). By fitting the folding stock, it would no longer meet the 60cm spec as it does currently, but would then meet the 30cm barrel spec instead. However if the sole differentiation between an air rifle and air pistol is being able to use and hold it like a pistol because the 30/60 rule doesn't apply, even though it would be a long one and rather unwieldy, it could still be used that way.
I think the fact I'm having doubts, backed up by your caution as well, means the safe option would definitely be to leave it as it is. I just really like the idea of the folding stock and in the grand scheme of things, it's hardly a weapon of mass destruction. God, firearms laws in this country are a mess!
Danny
My collection = Ratworks BSA Scorpion T-10 .177, HW100KS .177 (tweaked by me), PP750, Crosman 1322 and 1377