Thanks for the helpful suggestions chaps.
Temporary botch using party balloons (in the absence of sturdy enough rubber bands or knackered bike inner tubes) instituted.
Will phone Snells tomorrow to order a new spring and ask if they can fit it.
We're both wary of removing the rear brake pedal in an attempt to replace or reshape the spring, as last time we did this (after I dropped the bike in the summer and needed a new brake pedal) Andy managed to strip a screw thread! Snells had to rescue the bike with their van and use helicoils to get a new thread in. I'd be way too embarrassed to have to tell them I'd done the same thing again!