Just to bring this to a close, I'd like to thank you all for your suggestions, which, naturally, I completely ignored!
I ended up buying assorted technic bits on ebay (gears, thin beams, long axles, bushes - all the stuff I seemed to run out of, plus some random bits that came with them) and a second set of cables, with the idea of stripping off the outer sheath to make them more flexible (anyone tried this, by the way?), and then just accepting 'donations towards' for my birthday.
I have 2 or 3 ideas for projects which I don't need any new sensors or motors for, so at my current rate of 1 project/year, it'll be a while before I need anything like that.
Total cost £47.86 (€56.19, $72.52), in case anyone's interested....