Lime mortar outside / Cement plaster inside - causing damp?

[odilon]

We recently did a full rehab or our 1890's house in South London, including removing old cement render from part of outside wall, returning all mortar to original lime mortar (some had 1" of cement mortar on top), and new plaster on inside walls. A couple of the walls are now having some stubborn damp problems, and I fear the combination of lime mortar outside and non-lime plaster (I assume the original had been lime-based) inside is causing problems.

Can anyone advise on what might be happening?

Thanks!