i moved into a renovated 200 year old sand stone house some 3 years ago, when we first moved into the house we noticed a large stained area on the living room wall. we approached our builder (seller) who was renovating the house next door and he told us is would sort the problem. He raked out the damp plaster and tanked the area before re-plastering and this looked to have done the trick, however three years on and we have noticed a steady deprivation to the stone work on the outside of the house in the treated area.
Obviously i am worried that as no DPC was installed the damp is still rising and somehow attacking the sandstone.
PS the builder wants nothing to do with us and surprise surprise has since gone bust........and for the record next door has the same problem!