Hey Blue Up,
A licence is a licence! If you had a UK PPL 9 years ago it is probably valid for your lifetime; read the notes in the licence. In any case if you have subsequently trained and qualified for a JAR CPL and ATPL well done, all you now need to do is to renew (note that renew and revalidate have different and very specific meanings under JAR) the Single Engin Piston Class rating on your ATPL (this rating should have been brought forward from the PPL, if it wasn't get on to the cAA and get them do do it). This will require that you pass a skill test with a JAR examiner (if you havent flown anything under 100 tones for years you may need some training before you are ready for the test but that's up to you to decide). You may be able to find a dual qualified FAA/JAA examiner who can do your skill test which will also count as the FAA biennial however, it sounds like the FAA licence is issued on the back of your CPL, if this has the same number as the current ATPL you are fine, if not visit an FSDO with your JAR ATPL and get the new FAA licence issued as B200Dvr says. If the JAR ATPL is not UK issued your UK PPL still exists and can have its SEP class rating renewed in the same way.
In the long term (and I assume here that you live in the UK) you will be much better to get the SEP raing on your JAR ATPL renewed; once it is valid you will need simply to fly 12 hours on an SEP in the second 12 months of the two year period of validity, your LPC on whatever it is over 100 tons you fly will count in place of the required dual flight and the TRE who signs for your LPC can also revalidate the SEP class rating (although many of them are nervous about doing this because they dont know the rules well enough). Take a look in LASORS which is available on the CAA website (don't know the URL but Google will find it) and you can find all the rules in there.
Happy landings
3 Point