jjoe, all your previous flight time will count towards the NPPL. In addition, the accreditation requirements have changed since the time to which Fake Sealion refers - things are more straightforward these days
You will need to take sufficient refresher training which will also require you to cover those syllabus items which the UAS syllabus didn't include. Such as precautionary landings and operation from grass runways.
Including any previous flying, you must meet the mandatory requirements to have achieved:
Not less than 32 hrs flight time as pilot of aeroplanes. Which must include:
A minimum of 22 hrs dual, including an hour of instrument flying.
A minimum of 10 hrs solo, including 4 hrs of solo cross-country flying (that means pilot navigation, not just bumbling about on sector recces or solo GH trips outside the circuit!) including the qualifying cross-country.
You must take the Navigation Skill Test
before flying the qualifying cross-country (solo flight of 185km / 100nm including 2 intermediate landings at other aerodromes).
Once all training requirements are complete, including the PPL theoretical knowledge examinations, you may take the General Skill Test. You will also need to pass the Communications (PPL) exam and RT practical in order to include RT privileges in your licence (actually you will receive a supplementary 'FRTOL' licence.)
The 32 hrs does not include the time of the NST and GST.
As has been said, the navigation element will require accurate pre-flight planning using a navigation computer - 'MDR' flight planning is inadequate. There is nothing wrong with using the Standard Closing Angle technique appropriate for the aeroplane you're using - but many of the FIs don't understand it and won't like using it. So you might be stuck with some ancient 1950s technique "Because we've always done it this way...."
You will have plenty of airmanship as a result of your UAS days, but the civilian way is somewhat different and you will be expected to fend for yourself far more than was ever the case at a UAS. No ground crew to clean the windscreen, pull the aircraft out of the hangar, DI it and refuel it. Also remember that many training aeroplanes have seen better days, particularly regarding their avionics!
Don't forget that you won't have a dark visor, so take some sunglasses on sunny days if you don't want a bad headache later.
A NPPL/SSEA will be sufficient for flying both EASA and non-EASA aircraft until 8 Apr 2015, after which it will no longer be acceptable for EASA aircraft. If you intend to fly EASA aircraft after that date, you must have converted your NPPL to a LAPL beforehand. How this is achieved will be announced by the CAA this spring.