Thanks all for the swift replies.
I don't have SLMG on my licence, (have been flying the Vig sans CAA as a 'military' a/c), so it looks like I'll need to get the rating. I'm off to CGS soon for my C Cat though - so I'll see if that counts for anything!
SEP expires in September so if I got the TMG rating at the start of summer it should bring them reasonably into sync and still leave plenty of time to get the 12hrs in. Getting the SEP revalidated at the same could work nicely.
I better send the CAA a grovelling letter to enquire...