VERY BAD damp in 9" solid wall.

[RIFUR]

Original 1915 brick kitchen 'extension' 7ft. square approx. and 7ft. high. Both outer 9" walls extremely damp. Most of the walls had been previously bodged with Browning plaster which was in contact with floor. Hacked off all plaster to 7 ft ceiling level. Brickwork very wet/damp up to around 1 metre height plus salt evidence higher. Exterior ground level at same height as internal quarry tile floor.(not damp) No sign of a DPC. No gutter problems. Question: Creams and silicone injection ok but I tend to think that a solid moisture proof membrane will last a lot longer. Can I cut through the wall at appropiate level and insert a modern plastic DPC a section at a time, overlapping each joint? (assuming I can cut through the old morter bed, it seems quite sandy rather than mortar like) I realise I will have to insert mortar into the remaining gap. More work but maybe worth it?
Can you advise please. Agree or disagree!
Thanks very much. R.