How to stay legal
Your EASA PPL(A) is a lifetime licence which means it doesn't need any administrative action to keep it. It is only valid with an unexpired EASA or JAA class one or two medical certificate. You cannot use a LAPL medical certificate or an NPPL medical declaration with this licence.
Ratings within your licence may need revalidating or renewing in order to keep them valid. For example, your PPL(A) may include a SEP(land) rating that allows you to fly a non-complex aeroplane defined in the Single Engine Piston (land) category class. This privilege initially lasts for two years from the date of passing the skills test.
You can fly more complex aircraft within the class if you have 'differences training' signed off in your log book by a suitable instructor. And the UK has also made EASA licences valid on UK registered non-EASA aircraft.
In this example, to make sure that an SEP(land) rating does not expire and continues for another 2 year period, it can be revalidated by:
If the pilot also has a valid TMG (touring motor glider) rating, hours can count from TMG flights towards the 12 hours required for SEP(land) revalidation.
If the rating has expired you cannot fly as a pilot in command until it has been renewed. To renew your SEP(land) rating you need to pass a proficiency check with an examiner who will require a 'ready for test' certificate signed by a Registered Training Facility (until 8 April 2015) or Approved Training Organisation, which may mean having to complete refresher training with them first.
Your EASA PPL(A) can contain an Instrument Rating (restricted) (IR(R)) instead of an IMC rating, with exactly the same privileges inside UK airspace. The IR(R) can currently be added into an EASA PPL(A) if it is obtained before 8 April 2014.
An EASA PPL cannot contain national ratings such as SLMG, SSEA, Microlight class ratings, Microlight (Assistant) Flying Instructor ratings or IMC ratings. In order to have national ratings, the pilot must also have either a full national UK PPL or an NPPL, as appropriate for the rating desired. However, the UK Air Navigation Order allows microlights to be flown by EASA PPL or LAPL holders with a valid SEP(land) rating providing that suitable differences training is signed in the holder's log book by a suitable instructor.
An EASA licence cannot contain a non-EASA rating, and yet there are certain non-EASA aircraft that require specific type ratings, which by definition, cannot be contained in an EASA licence. The usual example quoted is a Catalina. This is a type-rated aircraft (it does not belong in the MEP class), and it is a non-EASA aircraft, so a Catalina type-rating cannot be included in an EASA licence. If you fly a type-rated non-EASA aircraft such as this, you will also need a UK national licence in order to have a licence which can contain the type rating. Remember this complication only applies to non-EASA aircraft that have specific type-ratings, if you have a UK-registered non-EASA aircraft that is in the SEP(land) or MEP class, the UK has legislated to allow you to fly it in the UK using an EASA licence with the relevant SEP or MEP class rating.