Pogs,
I think you're stuck with having to buy an NPPL to put the microlight rating on. At least you don't have to do any particular number of hours of training or any more exams - an oral test on microlight-related stuff prior to the skills test is all you'll have to do.
It's a nuisance, but at the cost of about an hour-and-a-half in a 152 it's probably not a show stopper I suppose.
Going the other way is much more expensive. Upgrading my NPPL SEP gained with microlight hours to a JAR PPL means paying for 35 hours of training; all to fly exactly the same aircraft as I already do, but without having to turn back mid-Channel!
By the way, is it Redhill you'll be flying the Jabiru from?