Thanks Whopity ...
Since leaving my orginal post I have trawled through Lasors and
http://www.nppl.uk.com/X%20Credit%20...20REV%2006.pdf ... as you say its as clear as mud, and full of conflicts ..
I rang AOPA this morning (who regulate the NPPL) ... the long and short of it seems to be that myslef as an FI (but not an examiner) can do it as a 'differences' type logbook sign off.
The student must then send a NPPL application form off to AOPA at Turweston asking for a new 'type validation' to SEP and pay £67 ... AOPA then authorise it and get the CAA to send out a new licence 'SEP class type' page ...
How on earth has the licencing system got into this state of affairs in the UK ... we are running almost three different types of licence with NPPL/JAR PPL and microlight licences ....
(Ironically if this chap had a JAP PPL issued with SLMG and wanted SEP then he would have to do a skills test .. ?????????).