I already knew how to get the BM apart, the belt themselves are an endless loop, but the thing that holds them doesn't seem to come apart.
Actually, with a bit of careful fiddling, I got one belt off.
I've measured one up, seems to be 30mm wide and around 425mm long, the tooth pitch looks to be vvery unusual, it isn't the normal 50%high/low type turns out around 4mm but its not really a proper way to measure them - the smallest tooth pitch I've seen is around 5mm, which would be reasonably close.
I think the thickness of the belt is likely to be the major issue.
I have not tried looking for a supplier yet, Wyco stocks Fenner belts so they might be an option.