[mark_in_manc]

Very useful, thanks - I think I'm running out of those grey ones in a white plastic surround, which have come in handy for all sorts.

(If you ever need larger ones, cut-up bits of carpet tile are very good - they tend to stay together on the backing, which provides a secure surface for gluing them on, and depending on how hairy the carpet is you can sometimes cut them with a hole saw).