Wallpaper paste contains a fungicide which is a mercury compound, not a good idea! Use PVA available almost everywhere, might need to dilute it a bit with water, say 50%. Paper based cat litter makes good papier mache, my wife won gold and 1st place for a mask made from cat litter at the New Fores Show.
I must endorse jomifl's post - wallpaper paste these days is dangerous stuff (unless you can get non-fungicide paste which isn't easy). Flour and water is good enough IMO.
thanks all, maybe ditch the paper mache one. Rowans link on how to make a mask looks fairly easy. Just hope they sell all that stuff at our local craft shop!