Sorry tell80, there are no water stains on the inside wall, therefore there's no point in showing any inside pic. The inside wall had insulated plasterboard applied, which will have a vapour barrier, so it likely wouldn't show through anyhow, even if there was any damp on the inside wall.
The higher extension render accidentally had building sand used in the plaster rather than plastering sand. There was a bag of each sitting there and the plasterer decided to use the closest one, even though I mentioned there was a bag of each. He had that wall done before I got back, so advised him to use the other bag to finish it. I think that's what may be causing the cloudy look on the higher extension render - it's also a bit of a rougher finish and I'm surprised he didn't realise. He had the bottom part done using building sand too, which may not be helping the issue?