I have a problem with mould in 3 bedrooms and now on the ceiling at the top of the stairs. I have informed my Landlord for the 2nd time and he has told me it’s due to me not having the heating on high enough or at a constant temperature. I open the windows EVERY day to ventilate the rooms but don’t want my heating on all the time. I would like to investigate this further and would be grateful if anyone could recommend a company that I could contact to come out and take a look. All the walls/ceilings that are affected are outside walls and feel very cold to the touch. Many thanks everyone!

If you are in France, check if there is something like Asloca

Only one solution. Get out. For your healths sake!

Get a thermometer, a hair hygrometer, and one of those color changing thermometer strips. Attach the latter to the wall. Such strips are available in brew supply or aquarium shops.

Then use a dew point chart or calculator to see if you have to get rid of moister and/or have to heat up the walls.

You might need if you do not want mold.

Also check this link. It has been discussed many times before on this forum:
https://www.google.ch/search?q=site:...+problem&cad=h

The natural tendency of a landlord, in such a situation, is to blame the tenant so the tenant pays for any remedial work.

I had a problem with mould in the apartment I was renting and after waiting for 3 weeks for someone to come and have a look, I ended up buying a dehumidifier- works perfectly and solved the problem.

Heating up the walls alone will improve the situation as the air will take up more moisture at a higher temperature.

To the OP - can you have the heating on, but at a lower temperature? Is it adjustable?

Most of the people are not aware that in a situation like visible humidity inside the apartment they can demand a discount of 20% on their monthly rent value as long as the problem lasts.
This is in the Swiss law and you can check it in the Asloca site of your canton.

To the OP. Get out! Mould in the bathroom is one thing. And common. But mould in the bedroom!!!

We had a bit of a problem with mold spots on part of the wall in two rooms in our old apartment (both exterior walls that become quite cold). We told the landlord and they sent a painting company over who first sprayed the walls down with some kind of bleach mixture, to kill the mold, and then re-painted the walls. I know there is also special sprays to use against mold on walls sold in home improvement stores here.
I *think* there may be laws in place in Switzerland regarding a landlord's legal responsibilities to take care of mold issues, but I'm not 100% sure. It does seem like I remember once seeing something about that once, though -- that the landlord is required by law to address the mold issues.

This sounds like a penetrating damp problem.

You need to get someone to look at the roof ASAP.