Was told this by the IRRE who did my last renewal, and have had it confirmed by a second IRRE. In theory at least, they should know!
Haven't found it in the JARs or LASORS yet, but I haven't had a good dig in the former for it - I'll have a look next time I have a high boredom threshold
If it's not true I've been flying SEP illegally for a while, albeit with correctly-signed paperwork.
Added later:
Right, just looked in the JARs, and JAR-FCL 1.245(c) says
1. (An applicant for renewal of an SEP rating shall):
(i) within the three months preceding the expiry date of the rating, pass a proficiency check... with an Authorised Examiner [in the relevant class]
The square brackets in the above denote text added in the latest revision of the JARs, 01/07/03.
So it looks possible that it was a loophole that has now been closed by amending the JARs. So ignore what I said regarding doing the SEP revalidation test on an MEP type.