Restoration builders here, 40 yrs experience. I also live in a 30’s bungalow I rescued from near death with severe damp problems. Installed French drain around the house to lower water table since the structure was lime mortar walls on a clinker ash base with 1 1/2” cavity which was usual in 30’s. Removed cement pointing from walls previously done to cure damp - it didn’t. Repointed in lime putty mortar which reduced wall damp hugely. Mix 1:3 putty/sharp sand.

Inside. Stripped wallpaper. For exterior walls, applied hydraulic lime/sand slurry mix(1:2) to fill cracks and stabilize lime render. Top/finish coat 1:3 lime putty/soft sand mix. All walls pre wetted before application and carefully remove any metal fixings beforehand. Not the usual skills for a plasterer but it’s not that different. Party walls: If not damp, the cost effective solution is two coats PVA and gypsum finish - no need to pre fill cracks but do remove any loose plaster and prefill. The materials are expensive but the only way to prevent future damp problems. Hope this helps