I'm back!
To answer everyone's questions:
1). I don't know why the decorator didn't tell me the plastering was shite. I think he's too nice a bloke. Apparently, the plasterer's insurance company is asking the same question and also asking for a C&G certificate to prove he's a qualified decorator. So I assume the insurance company won't pay out if he hasn't got a certificate because he's not qualified, or if he has got a certificate, they won't pay out because the decorator shouldn't have carried on with the work.
2) It was plastered over old lava & plaster (?? It's a 1930's house)
3) My "friend" said that his room was plastered perfectly.
4) I agree, I should have waited for the previous plasterer to become available. But I've been promising my 11 year old daughter for a year now that's she'll have her own room very soon. And I have everything else lined up ready to come in (decorator, carpet, wardrobe, bed, mattress etc).
5) I want a refund of what he charged me for the plastering. (£550) Plus the money for the wallpaper he ruined. (£40) Plus some of the money charged by the decorator.
5) I ALWAYS make lots of tea & coffee, offer biscuits and have even been known to supply bacon sandwiches.
Hope that clarifies some things.