The cheapest option may still be to renew the SEP, then do differences training on the microlight.
Proficiency check with an examiner will maybe be around £250, less if you can get mates rates on an aircraft and examiner.
Licence fee to the CAA to put it back on the front of the licence around £70
A few hours differences training will probably be £2-300 and you'll be good to go.
Comparing that to trying to get an NPPL(M), even with the allowances the BMAA will give you, it will likely take you longer, and you'll end up having to do a full skills test at the end.