I've a UK ATPL (JAA) and have not driven anything smaller than a 757 in 12 years. After e-mailing the nice chaps at the CAA, they phoned me (!) the very next morning and explained that I need to do about 1.5 hours in a C150 with an instructor in the UK and to then pay about £70 UK to have single-engined-land-planes added onto my ATPL. Something like an 1189 form (?)
They suggested forgetting the UK PPL altogether.
Hope this helps.
R.T.