@Fredvon4
Thanks for the reply. In this case I don't think the problem is caused by UV, these rifles rarely get to see daylight, let alone the sun! They are kept in a slip-case and shot indoors for the most part. The exposure to the slip on butt pad was measured in weeks rather than months, but the darkening is pronounced and obvious.
I am beginning to think that the problem may be more related to a release agent being left on the inside of the butt pad. Whoever above said wash them thoroughly and then talc the inside I think was onto the right thing, even better than talc would be fullers earth (used to have loads for NBC decontamination) which would help to neutralise anything nasty leftover from washing.
Ho-hum, do not want to have to refinish them, again in the case of the K31, and especially in the case of the Enfield which is stained, But it is beginning to look like the only option...
Bob