Having Just Watched The Film 'Dont Look...
ChatterBank7 mins ago
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For more on marking an answer as the "Best Answer", please visit our FAQ.They may need to use a special lime based paint. BEFORE sealing with PVA check this out as the walls may need to breathe - a PVA coating could, in the long term, present even bigger problems. If 'stanleyman' or 'buildersmate' see your question they will give you a very professional answer. They are both excellent.
I used to live in Spain and damp is a common problem because there won't be a damp course. By any chance does the outside have a higher level of ground against the outside of the wall than the inside?.
A more drastic step would be to remove the render in the area of trouble and have a look at what is going on.