In article , Andy
writes

There are many useful articles and a list of specilaists in all areas. Also
try you local conservation officer at the council. In my experience it
probably isnt worth trying to persuade a plasterer to have a go. Although
they would certainly possess the neccersary skills most will think that you
just want some builders lime (hydrated lime) added to ordinay gypsum stuff -
this is not what you want.

Try asking them if they slake their own lime - it doesn't matter whether
or not they do, its the response that counts - "Yer wot" is a bit of a
give-away.

J.
--
John Rouse