Afternoon all. Our house is a 100 yr old terrace, suspended floors(suspended 5 ft over dirt, i considered getting a cellar put in!)
and cavity wall.

Our neighbours (who have actually just moved out) went on the economy drive, triple glazing and cavity wall insulation etc. However the cavity wall insulation has now breached the air gap between the two leaves of the wall. In one point, in the bottom corner of OUR house, in a cupboard, there is now 1sq ft of damp to wet wall. presumably the fill may have spread out down at this bottom corner.

are there any quick fixes worth thinking about? i cant be bothered with structural works or talking to them about it (especially as they have moved away now).

I was wondering more about “paint on” type fixes if they even exist.
or would it just trap the water in the wall and make it rot?

Wont it be solid brick ie no cavity if it s 100 year old?
There is a temp paint fix if its not too damp, Glutenglas is a quick drying varnish type of affair or if you cant get that just paint it with oil paint. No paint will stop it coming back through eventually though

or would it just trap the water in the wall and make it rot?

Exactly that, I’d expect.

Old houses are supposed to be able to breathe, modern building materials can cause serious problems with damp due to bad ventilation (my house is the same vintage as yours).

It is cavity twang, i know because the outer leaf was falling off the inner leaf around the front 😯 . It a Let before, so had been let go to the dogs structurally.
another options i was thinking was to vent the cupboard into the floor space (which is massive, and has excellent air flow now we’ve sorted its ventiltion)

If the wall is that wet, then the lime mortar will also degrade over time and your plaster will also blow as they really really don’t like sitting in water.

You need to stop it first, and then work out how to get rid of the existing damp before it causes anymore damage. Neither of which will be cheap or easy. Venting into the floor sounds good, but it depends on how much water you are talking about, as the walls running with damp or just wet to touch?

Might be worth getting a specialist who deals with old houses to take a look, but you will need to find someone who isn’t just trying to sell you a delta membrane or equivalent kit, which will cost a fortune and only hide the symptoms.