If your cottage is stone built it is likely that the mortar used in constructing the stone is a lime mortar. This means that a lime plaster needs to be used over the top of it. Lime mortar doesn't set fully and is permeable so moisture moves through the mortar. You say the developer used 'normal' plaster on the walls, if by normal you mean a cement based scratch coat followed by gypsum plaster then this could cause problems. Due to cement setting harder that the lime below, this can lead to cracking and possible debonding of the plaster.
So in answer to your question a lime plaster should be used and perhaps it hasn't?
Feel free to email me if that helps.