There have been a number of occasions in which a cabin crew member has "helped" after one of the two cockpit members. Can't see a passenger being allowed to do this:
Flight Attendant Lands Plane in a Pinch /
Flight attendant helps land plane at O'Hare [Archive] - PPRuNe Forums
Air Canada flight attendant helped land plane after co-pilot breakdown: report - World - CBC News
They both had CPLs - but a bit different. I doubt that would happen in the UK though (and I doubt even more whether a passenger would be asked to do the same job!