Damp patch on internal wall

[Tiba]

Any idea what could cause this type of damp on an internal wall of a Victorian terrace?

The other side is vacant neighbours property

A leaking pipe perhaps and that looks like it could be mould growing on the damp patch. How long has it been like that? Can you arrange access via a relative of the neighbour?

I'd say either a leaking pipe in the wall or water pooling at the bottom of the bottom of your neighbour's wall due to a leak there. I'm not an expert by any means though.

upstairs or down?

there's a chance water is running down a conduit.

Are you safe to unscrew the socket and look behind?

What shape is the roof (pics please). Some Victorian terraces had pitched roofs with valleys.

I would get it sorted and not use that socket in the mean time.

It's been behind a cabinet so socket not used.
Damp patch probably been there since socket was installed.

We moved in in Jan and had house decorated and rewired at the same time.

Next door are renovating their house and have mentioned that their survey through up about the chimney flashing needing attention.

Could this be a culprit?

This is ground floor. Wall is the right hand side of chimney breast.

Removing socket shows damp plaster behind.

Roof is pitched, no gulley.
I will take picture tomorrow in the day light

If roof is the culprit would I not see damp patches on upstairs ceilings and walls on 1st floor?

If not, am I best to get a roofer round to sort this before fixing and tidying the plaster around the socket

disused chimneys must be ventilated top and bottom, or they will be damp. They usually contain two or more separate flues, because upstairs and downstairs fireplaces have their own. They may be build with next door's flues to form one big chimney structure. Each flue must be ventilated.

sometimes builders leave redundant pipes inside fireplaces when there has been a back-boiler.

ps
builders also like to leave old chimneys and fireplaces full of rubble and sand, which may get damp through contact with bare earth at the bottom.