stuart45 gave the answer to your difficulties: use a sand and lime render to 300mm above the signs of damp. Remove skirtings before knocking off the damp plaster. You've not responded with answers so its difficult to advise you further. But if you suspect that damp is penetrating behind the stair stringer then knock off the plaster behind the stringer (feel behind it for fungal damage), and merely render up to it - and try to render behind as best you can. i suspect that you have a cavity and if you do then open it up for bridging rubble inspection.