What’s going on with damp patches in my party wall?

Could be many different reasons, what's on the other side of the party wall, without looking at the property it impossible to say exactly what is happening, could be condensation. could be a roof leak depending on how they have sealed the separation wall between the property's, could be pointing, guttering the list is endless,what's up the stairs on both sides of the wall

If it’s like darkening of the paint ? It’ll be hygroscopic dampness , this is when the plaster ( not render) absorbs moisture from the atmosphere, technically it’s not damp. The only cure is to replace the defective plaster patches with render.

Could be anything really. Could be pointing from the external wall Leaking gutted causing penetration damp etc

Hello, You seem to have an issue with condensation. The brick walls of your victorian property have been either a) on the exterior side: rendered or re-pointed with dense cement pointing and/or concrete render; b) on the interior side, the walls have been rendered with cement based concrete "damp proofing render (with added polymers)" and / or gypsum bonding and gypsum plaster, and / or wallpaper glued on, and / or many coats of plasticised paints have been applied. These decorative treatments and renders are too dense and don't allow the brick structure (originally bonded with lime mortar) to breathe sufficiently. Over time the moisture level within the brick wall becomes too high and water will push out through cracks or form large damp patches on the walls. This is a problem usually associated with ground floor and lower ground floor areas of brick buildings. Other reasons for damp patches appearing can include: Water ingress through insufficient drainage, obstructed air bricks, drip leak in house pipes or water pipe connections on property, leaks in water pipes located in the street. Or a combination of all of the above. A site visit is required in order to assess the issue, to look at the surrounding area ie. ground levels) and to determine what materials are present on the walls.