we just bought a basement flat in London with this problem. The only long-term answer is a complete tanking or damp course treatment professionally done. They will give you a quote, then if thats ok come in and do the work which for 3 walls took just over 1 week. The walls to treat are marked out then the old plaster is hacked off down to the brickwork. At this point if youre living in your home its really damp and humid. After this they inject chemicals into the bricks and create a membrane to protect the walls from any moisture. Once this is complete they then plaster over the walls and thats it. The plaster takes a few days to dry and you need to be aware that you cant actually drill or put nails into the treated walls in future as this will pierce the membrane and ruin the work. 3 walls cost about 1500 quid and you need to be prepared for a cement mixer in your home. Kiltox in Greenwich, London did ours so if youre in London I can recommend them. Hope this helps.