AF,
From the NPPL web site
http://www.nppl.uk.com
Cross-crediting licences and ratings to NPPL(SLMG)
3.1 Current pilots
3.1.1 NPPL(SEP) or JAR-FCL PPL(A) to NPPL(SLMG)
The holder of a NPPL(SEP) or JAR-FCL PPL(A) with a SEP class rating who wishes to obtain a NPPL(SLMG) will be required to:
(a) Produce log book evidence of having satisfactorily completed the differences training in a SLMG with a flight instructor;
(b) Hold a valid NPPL medical certificate.
Looks like you will need to do a couple of hours "differences training" and pay your £131 for the NPPL.