Mould after grant funded insulation?

Here’s a few links that have help us in the past with damp.

It’s on my Wakelet site.

https://wke.lt/w/s/7wNUdu

Have you considered using http://www.damp.co.uk the Dutch method of eradicating damp and mould.

I experienced the same problems on my property back on 2006.

Once I implemented the Dutch method of damp control/mould, my problems were resolved.

By all means look to your guarantee on previous works carried out also.

From the picture it appears that mould is also growing on the plastic window, which suggests that the issue is one of condensation, not water ingress.
Anything close to the wall, such as long curtains or furniture, reduces air circulation which exacerbates cold pockets, and condensation.
Are your tenants leaving the curtains closed?
Also, some mould preventing paints can make the issue worse. They work by waterproofing the wall underneath in a supposed attempt to stop water from outside coming in. But in doing so they can cause the vapour inside the house to condense onto the wall rather than get absorbed.
My first steps would be to shorten the curtains and remove the water proof paint, replacing it with a breathable paint. It may be easier to remove and replace the existing plaster in that area. This will also give you the chance to inspect the brickwork behind and test it for water content.

The age of the property suggests solid brick walls. You suggest they might be insulated. Tap the wall beneath the window. If it sounds hollow there might be insulation. If it sounds solid then it could be solid brick. Try to estimate the wall thickness. If it’s 9 inches it’s solid brick.

Now solid brick is a worse insulator than a double glazed window, so more condensation will happen on the wall than the window. Wet surface on the walls equals mould. Look at the mould on the window frames. A cold room, so any moisture will condense on the coldest places … the walls.

In our current home, with solid brick walls, we suffered damp walls when we first moved in 40 years ago. We insulated the walls. We dry clothing inside the house. We don’t ventilate the bathroom much. We don’t have mould because the walls are insulated and so are warm and the moisture doesn’t condense on them.

The electrical wiring looks dodger, clopped to the skirting.

Look at the skirting there is a screw or nail holding it to the wall, you can see the brown rust and stain beneath it. The nail is metal, a good conductor, in a solid brick wall. The head of the screw / nail is colder than the wood. Condensation forms on the coldest places, the metal, it’s often damp and it’s rusting.

Insulation gets rid of many problems, and it reduces energy costs. These are all reasons why government is raising energy standards for rental properties.

Insulation rules OK!

The whole building contains a certain amount of moisture in the air. This moisture will tend to condense on any cooler surfaces. Now if you add insulation in one part (e.g. above ceilings)then that part becomes warmer, so no condensation there. But then there is even more moisture in the air and it always condenses on the coldest surfaces. So adding insulation in one place could increase condensation elsewhere. It’s basic physics, even the government can’t change that!