I'm curious about something here 'though - looking at the NPPL website it says that all NPPL holders require a biennial flight with an instructor, but my understanding from what's come out of the BMAA seems to say that this is not the case for NPPL(M) holders
AIC W43/2010 details the revised revalidation requirements for NPPL holders, which actually were introduced in 2008 by AIC then included in the ANO update in 2009.
AIC W42/2010 details the revalidation and renewal requirements for microlight aeroplane class ratings. Para 2.2.1, for NPPL holders, then refers you to AIC W43/2010, which ultimately refers to the revalidation and renewal requirements contained in ANO Schedule 7.
patowalker, as bose-x says, you must be a revalidation examiner to sign a Certificate of Revalidation or Certificate of Experience.