Thanks for the advice. There will be a PA system.
Opted for the Carlsbro (not bought yet) as Fender have said there's no rumble 500 stock coming in till September. We've ordered one from a UK site who must have a back order coming in May, but just in case it doesn't turn up then my sons a bit stuffed. My local music shop sourced the Viper kit so I thought it would be a good back up just in case.
I can't seem to find any reviews of the Viper amp head. Carlsbro said it's been used by bands all over the world but I've no way of knowing how good the sounds will be.