damp in wall

[someonelse]

old 20th cent property.
Downstairs loo the paint is flaking off and brickwork crumbling.
Solid concrete floor.
Walls on each side of the loo are internal.
Wall at rear behind loo (much less damage) is external.
Pictures below.

Was thinking of just using plaster to fill holes, use a damp-proof paint, and redecorate.
What better long-term treatment is recommended?

 

[JohnD]

This room contains pipes, drains and water.

Can see any signs of a leak, other than the excess water?

Have you got a water meter?

What are the pipes made of?

What damp, and pipes, can you see in the adjacent rooms?

 

[someonelse]

Hi and thanks.
No leaks visible from the cistern or bowl.
The toilet outlet goes straight down into the floor (as opposed to backwards throught the wall) so I suppose there may be a leak underground. No way to know without digging I guess.
No water meter, no.
Pipes are plastic.
Other side of one wall is a cupboard with no pipes and no signs of damp
Other side of other wall is utility room: mains supply pipe rising up through floor, and washing machine with outlet going through rear wall. No signs of damp.