The message
from Chris Hogg contains these words:

Much cheaper than Claybreaker is a bag or two of plaster from your
local builder's merchant or d-i-y store. Basically the same stuff and
does the same job.

But in that case get sirapite (pink), not one of the
platerboard-covering (grey) ones.

--
Rusty Hinge
http://www.users.zetnet.co.uk/hi-fi/