This is a very grey area because 9 times out of 10, the cause is tenant's lifestyle.
My tenants reported a serious problem in my rented property especially their you best daughter room. I was surprised as this has had never been an issue before. They insisted it was to do with problems with the roof and wall.
I went for a visit and the issue became clear. The bedroom, which is small was overpowered by furniture against every wall. There was clothes drying on the radiator that was full on and the window was closed. The problem was obvious.
There was also mould in the kitchen and I found out the dryer was a condenser one despite a whole in the wall to allow for a normal one.
I explained this but they wouldn't have it and threatened the environmental agency. They came and agreed with me. They left shortly after that.
After cleaning the whole house, new tenants moved in and funnily, not one bit of mould since they've moved in 4 years ago.
You need to make all efforts to avoid it. Don't overcrowded the rooms, leave some air between walks and furniture. Open windows for a few minutes every day. Yes it's cold, that's life.
Don't use a condenser eyer I less you have to, if so open windows when you use it. Get a humidifier.
If all those things fail (and record everything), then indeed,report to the environment agency.
Saying all that, if they've already had a roofer coming and quotes, they are probably reviewing the validity of the quotes and whether it is really the cause of the mould.