would appreciate if anyone had any tips on cleaning bits of hardened plaster from carpet?

I'd try breaking it up into as small bits as possible then combing it very gently with a hacksaw blade.

That also works if you have slightly scorched a carpet - allegedly

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bleach

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Plasterer in Coventry

Dust sheets work well

Stupid question from an ignoramus, but is this a water soluble finishing plaster? Try a damp sponge, after first knocking off as many bits as you can with a mallet/hammer as suggested earlier. Also try simply crushing it and sweeping up the dust.

Hammer, 40 grit sandpaper, hoover. Don't sand if the carpet is white

Let it dry, give it a good bashing with a lump hammer and hoover it up. Don't wet it.