Thanks for the above help - the more I hear the more questions I have! There are relatively few deep problems with the plaster. The first room I'm doing is about 4m by 4m and has a few screw/nail holes where the plaster has come away (all <2cm deep and <2cm by 2cm). There are a three areas...
Lots of really helpful advice on here - thanks to all those who have offered advice I've already benefitted from!
Have just moved into a house - 1930s construction. Most rooms had horrible textured wallpaper - looked like grains of rice in the paper which had just been painted over and over...