Many thanks for all the replies. I've discovered that they're also known as
"gluts" in Australia - presumably leading to the pleasing situation where a
company with too many of them could complain of a glut of gluts - and
"chocks" in this country.

That latter name was supplied by the helpful warehouse manager at my local
B&Q. "I think there's some in the skip - you'll be welcome to a couple" he
said, only to return, alas, empty-handed.

I think I'll try Jewson's tomorrow.