Antony has got it dead right.
I've tried heated, grips, gloves, waistcoats, willy warmers, everything for riding thru winter and the waistcoat is by far the best IMHO.
heated grips do help as in you can wear thinner gloves when its cold though, but thay work better with either a full fairing (not on a Multi) handguards, handle bar muffs, or 2 litre plastic milk cartons cut open, used as handguards, which look really NICE!
Silk gloves under some Hein Gericke lobster gloves are the best, and of course the heated waistcoat.