Answered my own question on this one but here for the benefit of everyone else is the word from the horse's mouth (pen?):
Appendix 1 to JAR-FCL 3.105:
(b) If a licence holder allows his Medical Certificate to expire by more than five years, renewal shall require an initial or extended, at AMS discretion, aeromedical examination, performed at an AMC which has obtained his medical records. (EEG may be omitted unless clinically indicated.)
(c) If a licence holder allows his Medical Certificate to expire by more than two years but less than five years, renewal shall require the prescribed standard or extended examination to be performed at an AMC which has obtained his medical file, or by an AME at the discretion of the AMS, subject to the records of medical examinations for flight crew licences being made available to the medical examiners.
(d) If a licence holder allows his certificate to expire by more than 90 days but less than two years, renewal shall
require the prescribed standard or extended examination to be performed at an AMC, or by an AME at the discretion of the AMS.
(e) If a licence holder allows his certificate to expire by less than 90 days, renewal shall be possible by standard
or extended examination as prescribed.
Link to this is:
http://www.jaa.nl/jar/jar/jar/jar.fc....3.105.apx.htm
So docfly is partially correct. An AME can recertify medicals which have lapsed by less than 2 years but only at the discretion of the national AMS ie Gatwick.