It's so grey I don't care £25 enough to even warrant banking it, I found "If you have reclaimed PPI, this is likely to have acknowledged the debt."
So by even the simplest of actions in which although you don't directly acknowledge the debt in black and white, the 6 year SB gets reset at the point of acknowledgement, by banking the cheque you are clearly acknowledging the debt did exist and you were involved, otherwise why would you accept the cheque?