All the rooms in the house are painted, with a matt paint, and a couple of them especially the hallway needs re skimming all together, i have only ever rendered a wall from Bare brickwork, with the scratch coat, second coat, devil float then first layer of skim, then second. I have never come across reskimming over an allready skimmed and painted wall. So just checking before i go to B&Q , if it is ok to uni bond the surface to allow suction, then just apply the skim ? Just so i know exactly what I need and I'm able to do it, thanks guys . . .

Unibond do a special PVA which will allow you to do this with confidence. Personally, I ieither use a textured surface remover to cut through the emulsion, or just PVA 1:1 twice, leave to tack over, then skim. Prefer not to do it though.