So, think I've found the culprit of our damp patch indoors..
The patch lines up perfectly with the position of the gas meter outside, after it rains it gets worse inside
I reckon the gas meter is trapping water behind it at the bottom, saturating the rendering and coming through inside.
Whats the solution here? Silicone the gap between the gas meter and the wall? I might bodge a water proof cover to stop rain going down the back of the gas meter to test, but I have a feeling water will naturally condense behind it too..