Mechanical heat recovery ventilation damp problem?

67 on a dehumidifier is quite high.
But do check that the reading is correct.
I had a very damp room with readings with a new dehumidifier if 77(rising damp in walls which were soaking) , it took month to get to 67, and a further month to get under 60 ( walls dry and nice feel in room, but we switch it off overnight and it's 67 the following morning) and then I gave the unit to my daughter, buying a replacement which said the room was 48. Which I would have accepted if I hadn't had the other dehumidifier the day before saying it was 67.
Sent the new machine back and got another that says 67, reducing to 58 but the end of the day. -- check that the dehumidifier is working properly before embarking on another option.

Dank smells = open the windows????

You don't say how old your flat is.
Retrofitting a full HRVU is very expensive and maybe not worth the extra expense, however in the 40+ flats (studios, 1 bed & 2 bed) that have ECH I have installed the Nuaire System 2000 Flat Masters positive ventilation units positioned on an outside wall and the incoming air ducted to the hallway which have proved to be extremely good at eliminating the condensation and subsequent mould you refer to. These require the internal filter replacing every 2 years or so depending on the quality of the incoming air.
For my top floor flats (yours) I used the Nuaire Dry Master units which are located in the loft space and again ducted to the hallway, i.e. to a central point in the property, filter change every 5 years or so.
You must ensure the window vents are permanently open and the tenants do not interfere with the dry air entering the property, e.g. taping over the ducting in the hallway.
A dehumidifier does not solve the root cause, they just treat the problem.
In order to ensure the fresh dry air is circulated round the entire property I ended up drilling a 50mm duct above each bedroom door as the bedroom doors are always closed during the night and some condensation was evident.
Prior to fitting these PVs in 2008 I was constantly plagued with condensation and mould issues and have saved £000s on redecoration costs. In short, they work.

I'd wipe down the walls and windows which may have mold spores on them. Maybe the carpet needs washing too

I'd also look at the pointing, gutters and roof felt which could be allowing water penetration. Water could be getting in if there's a problem when the flat's empty (ie not tenant lifestyle related). It could be that the south side is also wet but manages to dry out in the sun before it penetrates inside

Slooky, it sounds like the company are trying to sell you a whole property positive pressure ventilation system, but you are looking for individual room heat recovery vents.

I had an individual HRV installed on a ground floor bay of a studio, which had always suffered from high levels of damp. HRV unit did the trick. Can't tell you the make but plenty of suppliers, so it's just a question of finding one which can process the volume of air required for a sensible price. As they are for bedrooms, you probably want to consider quieter models.

Reply to the comment left by Chris Bradley at 02/01/2024 - 10:43
Dehumidifier is working well. Collecting a huge amount of water. I have no previous experience of humidity sensors so I bought 2 identical ones for the flat and 2 different brand for my house. I had them all set up in my house and the 2 which I have subsequently put in the flat read slightly lower. The walls aren't wet to touch just cold. There is no sign of damp ingress because we assume you would see staining and there is none

Reply to the comment left by Judith Wordsworth at 02/01/2024 - 10:53
I fully sympathise with tenants not opening windows. The building is 1850's with timber sash windows and in the winter they don't behave well. Lovely in the summer. They are maintained (such a faff). In the winter they swell and it is a skill to open them evenly otherwise they jam and I have trouble myself closing the top sashes correctly when locking them

Reply to the comment left by David Moreton at 02/01/2024 - 11:10
The flat is 1850. Solid walls.
I believe I spoke to nuaire and they didn't recommend fitting the drimaster in the loft because they didn't like the fact that we have an internal water gutter. The loft is well vented in the roof tiles and because we have an internal gutter there are openings in the roof so the air rushes through. We have looked at the systems with the heater but many owners do not recommend the heater because of the expense. This is why we were looking at MHRV rather than PIV. The tenants would then never have a need to block the vents. I understand that although the Mechanical heat recovery ventilation unit is more expensive to buy it is cheaper to run because of the heat exchanger. Fitting is not a problemy husband will fit it so we save on the fitting cost. We just want to get it right

Reply to the comment left by JB at 02/01/2024 - 11:29
We use the zinser mould spray. We don't wipe the walls because the spores will spread. Wetting the wall with the mould spray prevents airborne spread. The walls are not black they merely go a green which some people would not even notice. Carpet is fine and the curtains have been washed just in case (they are now 2 inches shorter!)