This reply is to both Chocky and Gypsy Moth....... I would remove the old wallpaper paste residue if you are planning to skim the wall/s over. That way, you are working onto an un-contaminated surface. You are then ready to PVA and skim if that is your plan. Tape over any cracks in the old plaster, remove and fill any loose bits and pieces that need it, and you're ready to go ..... It's all in the prep every time.