What's causing the damp in these photos?

[stem]

but the ceiling only having 150mm,

That was just my guess, it might be more, I'm going to nip over next week and check.

as of this June, you can no longer rent out a property that has less than an EPC rating of E, so Fs, and Gs

It's officially rated as D but the assessor was incompetent. See here.

 

[Doggit]

He wasn't necessarily incompetent, but he was working along predetermined ideas of how a system should work, and there's an argument to say he was correct. Whilst you may have a zoned system, a TRV will determine when best to switch off a rad, so each would work independently. If the sun shone (even in winter) is could warm that room up more so than the room next to it. D is an okay rating, but whilst it'll be good to upgrade the loft insulation, I doubt if it'll solve your problem.

 

[stem]

Whilst you may have a zoned system, TRV will determine when best to switch off a rad, so each would work independently. If the sun shone (even in winter) is could warm that room up more so than the room next to it.

That's what it achieves. Every room has its own dedicated room thermostat and motorised valve. So every room is totally independent. There's even a row of switches in the kitchen that allows each room to be switched to be 'on' 'off' or 'timed' individually.

The inspector said he only saw one thermostat, so missed all of the others. In an empty property with plain walls they were pretty visible. as was the switch panel in the kitchen. Sorry but that makes him incompetent in my book.

but whilst it'll be good to upgrade the loft insulation, I doubt if it'll solve your problem.

I fear you are right.