I'm trying to get to the root cause of dampness in the front wall of a small Victorian mid-terraced house, built around 1870. I've owned this house for many years, but the damp problem is relatively recent. Here's the history:

Bought this house in 1991. The house is unusual for the era in having a cavity wall at the front, and the cast iron wall ties were failing- these were replaced with stainless steel ties. I believe that particular area had unusual local building regulations, and wall tie replacement has been big business round there. There is no evidence of a DPC, but the bottom 25cm or so of the wall has been rendered outside. The surveyor's report didn't point out any damp on the front wall at the time.

In 1993 the council raised the pavement level outside the house so that the pavement now slopes towards the front of the house (it has no front garden). The level where the pavement abuts the house is the same as before but with the addition of a drainage channel along the front wall. The street has a gentle slope and water from the channel can drain away across the pavement further down. There is some water splash from the pavement sloping the wrong way but this didn't cause any noticeable internal damp while I lived there.

Moved to a different area in 2002 and let the house. My last tenant (who generally looked after the house very well) started pointing out internal damp at low level on the front wall in 2014. Eventually I had the front wall repointed (it needed doing), along with raking out the cavity and fitting a couple of extra air bricks. I also checked that the window sill was properly sealed. However, we're now 4 years on since that work was done and the damp hasn't gone away. It is evident as small blemished areas of plaster and a little mould, and the wall feels damp to touch.

I've gone over the wall with a cheap moisture meter (I know it isn't accurate but gives an idea at least) and am getting meter readings around 30-35% near ground level and minimal (less than 5%) higher up. The skirting board on this wall gives around 25%.

So does anyone know what's happened, and why it's taken over 20 years to appear? Many authorities tend to dismiss rising damp, but could this just be a rare case? Condensation caused by the tenant's lifestyle might be a possible cause, but I'd have thought that would manifest itself in more than one place and not just at low level. Suggestions welcome