While the builder bought the Celotex boards, I am fairly sure that these were bought from trade at the time he needed them. They were sat in the garage for a couple of days before use and looked brand new. I have pics of these somewhere too.
The order of doing the conversion was a bit back to front as they were trying to complete before xmas, so that wall and a couple of others were plastered while the garage door was still on, so was cold with no heating for a couple of weeks. So @worthwoods probably has it right.
It's all the way down the External wall, not just at the window end. Worse at the joins.
I have wiped down the walls with mold solution and prepped ready for painting. I am going to leave a few days to see if anything more comes out. The heating has been on in that room now for a good couple of weeks so the room is now aligned with the rest of the house.
Thanks all for your feedback. I will report back at the end of the week in hope it helps anyone else.