So to summarise, this is a standard Victorian terrace and the damp is along the rear external lounge/diner wall, to a height of 1m, and extends sideways along the internal partition? And from inside this is effectively one wall across the back of the lounge/diner?
You need to eliminate various possible causes and determine most probable causes and then concentrate on a most likely source of the dampness.
Assuming you've discounted a leaking pipe, there is no raised external ground against the wall, and gutters are not leaking and no obvious dampness to the external wall or damaged brickwork, and no obvious bridge of any DPC
So from the lounge, if this damp is at a constant height all the way across the wall then its moisture coming up or a cold [different] material in that part of the wall causing local condensation
If the damp is higher where the wall is external and tapers off as it becomes an internal wall, then moisture coming from outside is more probable
If the damp has a definite line, nearly level, of a dark brown/yellow stain and possible some white salts, at its uppermost level, then this is more likely moisture coming up.
1m high dampness is more likely damp coming up (failed or no DPC) or condensation. It seems that you have some render on the internal wall.
This can lead to two possibilities (based on my experience of these properties)
1. This type of rendering work either hides the damp completely and it never penetrates through to be an issue, or in extreme cases damp can rise behind the render (which is typically 900mm high) and shows at about 1m high.
2. Hard render creates a colder surface from the rest of the wall and condensation forms on it. Dampness behind the render acts to cool the wall too and assists condensation, or some evaporation occurs too from some penetration of the render. The nett effect is that the band of render shows up as damper than other parts of the wall or room
Now, without a site visit or additional tests, its had to say which is occurring. The symptoms of each of these are different and need to be investigated on site, and there could be a bit of both
A test is available to drill the wall and analyse the dampness within (not just on the surface) and this can tell where the moisture is coming from. Or it may be possible to look at it and say which is more likely
If the cause is obvious and can be removed (ie leaking pipe or raised ground) then removing it will remove the damp. But if the damp is the result of a failed (or no) DPC or failed render (or end of life) then work has to be done.
Even if the cause is solely condensation, then unless you have symptoms/issues elsewhere in the property, then its not really something to be dealt with by opening windows etc, as the problem will still remain
For typical remedial work, the render will need to come off to 1m height and either re-done or a different plaster is used. At this time, either way, it may be prudent to inject a new DPC into the wall as this is a relatively small part of the cost of the whole job