Walls

Can anyone advise.....  We live in a small stone house that was renovated in the 'rustico' style approx 15yrs ago.  It is built into the side of a hill and consequently suffers from the usual damp problems which are controlled by virtue of the fact we can keep it well ventilated and heated when requited.  However, the internal rendering is suffering quite badly from cracks / blown plaster and peeling emulsion paint.  On investigation we find that the underlying stone walls were not sealed before the rendering was applied. Before we start to chip off all the loose stuff has anyone any advice/ experience of using PVA sealant?  Is it available here and if so, what is it called?  We have in small areas tried 'flexible filler' - no good and cement - wont adhere to the stone.  Any advise much appreciated.