I had a similar situation
I already had an FAA PPL issued on the basis of a German LBA PPL. In the pre-JAA days, it was much easier to keep a UK CAA PPL current than the German counterpart, so I did a full UK CAA PPL licence (i.e. not issued on the basis of the German one) and let the German PPL go out currency... Now, my original FAA PPL was valid on condition that my German PPL remained valid, and this was not the case.
When I tried to get a "new" FAA PPL on the basis of my current UK CAA PPL, I was told that it was not possible as I had already been issued with a PPL on the basis of another licence and this could only be done once according to some obscure FAR!
Can anyone can help me out on this one?