Where to start with damp and mould

[Littleredwagon2308]

We've got mould growing on many of our walls of our owned property. How do I go about finding out what the cause is? It's quite widespread. Could it only be because of lack of heating and ventilation? We open all the windows once a day for minimum 10 minutes and usually for 30 minutes (often longer). We didn't switch the heating on until the temperature was below 20C. Should have we have switched it on earlier? At night it's set to come on if the temperature drops below 16C.
Who do we call for advice? Do we call a builder? A structural engineer? Who?

Also, the mould is also behind the kitchen cupboards. What do we next there? Remove the fitted kitchen? Paint walls with special paint? Install insulation board? Install a heater under the cupboards? Drill ventilation holes?

I just don't know where to start! 🙃

I'd say the OP has more going on here than not heating her house sufficiently. My house is older, I don't have any added insulation in the walls and 2 of the rooms (each having 2 external walls) haven't had rads for a year, so are currently not heated. Thermostat is generally at 18 and lower at night. Zero mould, apart from a tiny bit on the bathroom ceiling if it hasn't been ventilated enough.

70 degrees? 21 C? 😮That must feel like a furnace, unless you walk around in the winter in a T-shirt and very little else. And up here in NW England, in an old house like mine, even carefully insulated where possible, it would cost more than an extra £300. I shudder to think what it would cost.
We usually keep ours at 17C, sometimes 18, during the day, higher for an hour or so in the evening when we sit watching TV, and colder at night ( no point having it on hotter when we have a window open anyway).
No damp.
If we were older, or in poor health, we'd have it hotter, of course- we turn it up when vulnerable people visit. Our surveyor suggested 18, and that if we wanted it hotter, just to heat the room we were in.
But we do ventilate it - a lot more than most people seem to do. If you get rid of the damp, by ventilation, sorting drains, all that stuff, the same temp. feels a lot warmer. When we first came here it felt colder in August than it does now in freezing winter weather- because it was damp. I had a duvet and two heavy blankets in August- don't need blankets now even in the worst weather. (Well, maybe one on the coldest couple of nights of the year.)
What we have been trying is turning the main thermostat up above 18, but setting rooms individually using thermostats on the radiators, according to use. I think that is working better- as even 18C on the main one often left the colder rooms lower than 17. Hopefully when we insulate under the lower floor the discrepancies will even out a bit more.