As per subject, what's the knob on top of a tap called? We have two
perfectly serviceable taps on our washbasin but one of the plastic
knobs is disintegrating - I'd like to get a replacement.

I guess the whole of the tap, as in the insert that can be removed
from above wouldn't cost all that much but even then I don't really
know what it's called.

They seem to have a fairly standard size serrated (I know that's not
the right word) shaft and are fixed by M4 screws.

--
Chris Green