You've got two components already - the Sansamp and the cab - you could always just look for a power amp?
Failing that, you can always find bargains on Gumtree (more so than ebay I find). Older Fender Rumbles sell for peanuts for example. If you don't mind heavy stuff, and travelling a bit to collect, you can always score a bargain. Otherwise, you're probably looking at trying to save up maybe £150-200. For example £150 could get you this currently for sale in the For Sale section.