I've now dismantled the bass to have a look inside. It looks reasonably clean to me, and doesn't smell musty - surely a good sign!
The leather valves curl significantly, but the reeds themselves don't look too bad to my untutored and unpractised eye.
The control box is held on by five brass screws, and also doesn't look too bad inside.The springs are firmly in place and quite strong. The buttons are simply screwed quite roughly into the valves,which look as if they're made from card sandwiched between two pieces of leather.