Re: Plasterer's looking for more money.
The guy is definitely chancing his arm - call him up (or call through to his voicemail if you aren't good with confrontations) and tell him in no uncertain circumstances that he's not getting another penny.
It's the same in any business - you asked for and agreed on X amount of work for Y amount of money. Just because he brought Z into the equation, that's his problem, not yours.