I have at the moment a CAA lifetime PPL with a Medical declaration and the licence is up for revalidation in November. (SEP)
No - the licence is lifetime, it is your SEP Class Rating which will be due for revalidation in November.
My understanding is at this point; my privileges are restricted to that of an NPPL, ie no night, no IMC, now lapsed anyway and requirements to fly abroad etc etc
Correct - your privileges are currently restricted to those of the NPPL(SSEA).
my question is what criteria should be used to revalidate
As you are flying under the exemptions of ORS4 No.816, you will not be able to revalidate your SEP Class Rating unless you hold at least a JAA Class 2 Medical Certificate. This change of regulation was introduced by the CAA last autumn. Otherwise you will need to apply to the CAA for the inclusion of an SSEA Class Rating in your pilot licence - including the issue fee.
If you have flown insufficient hours to meet the SEP Class Rating revalidation by experience criteria, you will need to fly an SSEA GST with an Examiner. This is actually rather more simple than the SEP revalidation LPC though.