condenstation / damp problem

[low drains drifter]

There certainly does appear to be a lot of condensation.
First, I'd identify the pipework outside and reassure myself that there are no leaks. That means outside and where it passes through the cavity, although that would be difficult to identify and perhaps beyond a DIYer. It looks like there is a pipe pointing downwards. What is this?
There's also an overflow. Is this flowing/dripping water?
It looks also as though the dpm has been breached.

Does your neighbour suffer any damp/condensation on this wall upstairs?

Once you've assured yourself that the moisture/damps is not being caused by water from above or damp from below, I'd definitely install at least 2 vents through the external wall, one low, and one high. Perhaps low behind the front door, and one at high level by the upper door.

the pipe pointing downwards is an escape for the pressurised boiler system
the overflow is the central heating flue

we're the flat that is upstairs but the ground floor neighbour does not get damp

 

[foxhole]

Do you dry clothing indoors?

 

[low drains drifter]

Do you dry clothing indoors?

it's a flat that we rent out. But I can see where you are coming from though. I can't say for certain but I never see any clothes being dryed inside. I think I would have condensation in other parts of the flat if drying wet clothes on a radiator was the problem.

 

[^woody^]

I think I would have condensation in other parts of the flat if drying wet clothes on a radiator was the problem.

No it does not work like that. Air holds moisture and this is deposited on any surface that is cold enough compared to the air - known as a dew point. This can be anywhere in the property, not just what damp clothes are.

That's why air is condensing in the cooler hallway, whilst the warmer air elsewhere is holding its moisture.

 

[low drains drifter]

I think I would have condensation in other parts of the flat if drying wet clothes on a radiator was the problem.

No it does not work like that. Air holds moisture and this is deposited on any surface that is cold enough compared to the air - known as a dew point. This can be anywhere in the property, not just what damp clothes are.

That's why air is condensing in the cooler hallway, whilst the warmer air elsewhere is holding its moisture.

yes, I can see the logic there