Can I remove the top layer of wallpaper only if it is not vinyl paper. I don't think the wall has ever been plastered, just plasterboard underneath and I don't want to have to plaster?

Sure you CAN take it off. The easiest way is to put a little fabric softner in a bowl of hot water and wipe this on the wallpaper. Let it stand for a couple minutes and the paper shoud peel right off. Do a small spot in a corner or low so you can tell if it is going to hurt the wall or not. If not, repeat on a larger scale. We recently did this in our kitchen and it takes off the glue as well... Good luck!