Many thanks for the responses.
The original DPC is a couple of courses above drive level and visible still outside the house. It was put in as a condition of mortgage about 25 years ago.
The house has 9" walls with no cavity.
I'm kinda sceptical about rising-damp, as the under stair area has efflorescence a couple of foot from the ground, with non really at ground level. If I thought replacing the chemical DPC would help, I'd take that on myself, though IMO that would be the last thing to try.
My theories are:
Condensation - The areas affected are in non-heated spaces and I reckon humid air from the kitchen has found its way into the spaces over time and condensed on the walls. We were not in the habit of using the extractor fan in the kitchen, unless windows started to steam up - yes that was a big mistake.
Penetrating damp through the weak old external render reaching the brickwork. It seems that the original mortar in the wall is quite absorbent. Not sure what it's called, but it's kinda black and friable. I'm therefore planning to point the affected internal areas, where possible, as a starting point. I'll have to get someone to do the external render, which needs replacing, whether or not it's causing the issue. Maybe the exterior can be repointed at that time, while the brickwork is exposed.
So right now, a possible plan of action is:
Point brickwork internally in affected areas
Replace external render
Re-plaster affected areas with renovating plaster - maybe add a vapour barrie (I need to research that one)(?).
Add heat sources/ventilation to the affected areas. I should have installed a mini-rad in the toilet years ago, but maybe it's also an idea to add a mini-rad under the stairs, to keep it at a minimum warmish temperature and just stop the area getting freezing cold, as it tends to.
Thanks in advance for any comments
c.