I did much of the certification of the Vigilant T1 for the Air Cadets. It has an MTOW of 908kg, whilst the JAR-22 (and UK) definition of a motorglider states an MTOW of no more than 850 kg. For that reason, personally, I've always regarded those hours as light aircraft hours, not SLMG hours. The CAA have accepted that when they've seen my logbook, although I didn't specifically draw it to their attention.
However, I know that other people have logged hours in a Vigilant T1 as SLMG (G109b) hours and had that accepted by the CAA. So, I think that you'll probably get away with it. It's worth looking on the CAA's website at
www.srg.caa.co.uk for the leaflet on gaining an SLMG license to check.
As to cheap schools, or schools at-all, I'd suggest giving the British Gliding Association a call (sorry I don't have their number to hand, they're in Leicester if that helps), I imagine that they'd be glad to send you a schools and clubs list.
G