To update, I got a quote for replacement bellows via camerabellows.com who quoted me a reasonable £80 + VAT. However. a work colleague suggested trying rubber solution (the sort that comes in cycle punture repair kits mixed with 'lamp black' (whatever that is!). I tried micing it with some 'Photo Opaque' which was in the bottom of a box of photoraphic bits I'd been given years ago - it seems to work fine! Cost = 0!