Forgive this question, i feel slightly embarrassed asking it. Jockey Wheels labelled ‘top’ and ‘bottom’ or ‘upper’ and ‘lower’….what is the difference between them? Can i be really radical and put a spare upper into the lower slot to replace the worn existing one? Or will i die if i perform this act of heresy?

Upper generally has a little bit of float whereas the lower typically doesn’t.

I’ve noticed it can have an effect on shifting when I’ve tried it before.

New ones needn’t be silly expensive. BBB ones are ok and cheap enough on ebay.

Or get some Tacx ones for a just over a tenner (mine were a tenner a few weeks ago) from Drover Cycles on Amazon. http://www.amazon.co.uk/gp/product/B000OYJUJY