I appreciate the advice, that's what I did in the end. I couldn't be bothered with the whole eBay return fiasco again so I opened it just to have a look and this made a difference to the problem. So I disassemble it, cleaned all the parts and reassembled it. The unit looks almost new with no dust of gunk inside but I think one of the tiny little gears must have been out of position. Reseating them all solved the problem. Works like new.

Think what's expensive?