So that's what the fuss was about! I have a compost bin which looks superficially similar, though it is just an ordinary bin of no particular value, and with no special innards. It was given to me by a plotholder who was leaving the site some years ago. The thief subsequently claimed that it was 'hers', it was special and valuable, you couldn't buy it, etc., etc. I never saw what she was going on about but she must have heard something about these things. She never got any of my bins, fortunately, since I fill them up the moment I've emptied them.