Three types of wall surface - plaster direct on stone, lath and plaster, and plasterboard. In all cases, savings could be made by adding more (cheap) sand to the (more expensive) lime. Even early plasterboard had a sand content, unlike today's gypsum boards. The extra sand makes the plaster more crumbly with age. Check that it isn't the wallpaper that's holding the plaster to the wall, as is often the case!! Otherwise, you can harden the whole surface by sloshing on a 5:1 water/PVA mix and letting it dry thoroughly. This will allow you to fill in dents and holes with E.G. Polyfilla, giving you a smooth surface for papering. But this won't solve the problem of the crumbly underlying stuff. The only real answer to that is strip it off and have it all renewed, alas.
BTW, Fischer wall plugs from most DIY stores are far superior to Rawlplugs...
http://www.tooled-up....roduct.asp?PID=159258