Concrete/cement interior wall

I would recommend to plaster the wall. Strip as much as you can, apply Blue Grit (a high-performance, ready-to-use primer with added aggregate, designed to provide a strong mechanical key for plastering) and decorate the wall (mist-coat + 2 coats of paint).

If you are wanting cheap effective then I'd recommend using some ready mixed filler, you can buy it in large quantities now and apply with a roller, sand for smooth finish and then you can apply paint or paper straight to that.

If the wallpaper has come off and the wall underneath is concrete or very old plaster, the first step is to make sure the surface is sound and clean. Scrape off any remaining wallpaper paste and lightly sand the wall. Any cracks under the window should be opened slightly and filled with a suitable filler, then sanded smooth once dry. If the wall is very rough or uneven (which is common in older properties) you have two options. The best finish would normally be to have the wall skimmed with plaster. Alternatively, you can prepare the surface with a bonding primer such as blue grit and then fill and sand the wall before decorating. Once the surface is smooth you can apply a mist coat followed by two coats of emulsion. Older walls can vary quite a lot, so sometimes it’s worth having a decorator or plasterer take a quick look to advise on the best preparation.

PVA OR BLUE GRIT WALLS AND PLASTER.

70s wall paper is a good look so is primed concrete!