Hello, we have a couple patches of damp in our Victorian solid wall which seem to be getting damp. We've had the drain at the front checked + a crack repaired a few months ago, but I don't know what else could be the cause. I don't believe it's condensation as we have good ventilation & heating indoors. Both patches look like water/damp patches. One of the patches is midway up the external wall, the other is a small patch (around 10cm x 10cm) right at the bottom of the external wall (same wall, but opposite sides). The wall has been plastered and finished with clay on the inside. The outside we didn't change, and it could be an unsuitable material causing the issues, but I'm not 100% sure.

I'm at the point where I just want to hire someone to come look and tell me what could be causing the issue, but I don't know what specialist would know this (familiar with old buildings, the material on the outside, etc). Does anyone know of someone recommended in London? We are in a converted ground floor flat, so this room is set to become my baby's room, and before we re-decorate, I really want to solve the issue once and for all. 

Dominic's suggestion sounds a sensible first step. Perhaps plastic film woud work as well as glass?

My first thought was SPAB, and indeed they suggest calling their Technical Advice LIne to find a contractor or surveyor in your area who is suitably qualified/experienced. They also mention the local Conservation Officer if you are in a conservation area.

https://www.spab.org.uk/advice/technical-advice-line

If you send me the dust from a few holes drilled into the walls where they look damp and one from a dry wall I can scientifically analyse them for % moisture content.

I already suspect hygroscopic surface layer(s) and no damp in the walls but let’s put my theory to the test before spending money on unnecessary work.

Sorry for the late reply, I thought I'd get notifications but didn't - there is nothing over the clay, the wall on the inside is entirely breathable (including the build-up plaster under the clay finish). I will tape a bit of cling film to the wall to see if the moisture comes over or below. I am almost certain it's something external though (though the damp may now be additionally causing condensation as the wall is wet/colder).

Aside from this wall, we randomly have one other area in the house which has a damp patch on the bottom of the wall. In this case, the wall is an internal wall (but an old solid brick wall that goes down into the ground), so it doesn't make sense for it to be condensation. I *think* fixing the drain may have fixed this issue, but I can't tell 100% unless we drill the holes and assess for moisture content now, after letting it dry out a bit. The house is well ventilated, it is well heated, we don't dry clothes indoors, we have a strong extractor in the kitchen, dMEV etc. I

'll take some photos shortly when baby gives me a minute! I really believe it's something external which is why I would love if someone could come visit and assess the outside for things I may not be aware of.

These aren't the best photos because the areas are shaded by furniture and I don't have the best lighting, but hopefully gives an indication.

If you look at the photos with the green coloured wall - there's one small patch on the floor (about 2.5/3 inches wide), and one larger area about 2 feet up the wall. They are on opposite sides of the same (external) wall. On the other side of the where the larger patch is, is a drainpipe. Initially, both of these appeared as only a darker (damp?) patch, but now looks like either salt has appeared, or mould has grown. Also looks like some salt or mould in the corner of the wall.

You can sort of see that there is a shelf adjacent to this wall, but there is at least around 6 inches of space (I'm careful not to block external walls, to allow for air circulation). It is a terraced property. To note, there is nothing/no damp patches/mould around the windows, which to me also suggests it's not a simple heating/cold wall issue. We also very very rarely get condensation on the inside of the window. I think last winter I only saw it on a few occasions when it was extremely cold temperatures overnight. We have not seen that at all this season so far.

The last photo, with the lighter colour clay, is of an entirely internal wall, in the middle of the house (but the solid wall goes into the ground). This used to show a visible circular damp patch every time it rained, which makes me think the cracked drain at the front of the house was somehow letting water underground and to reach this area. Since we fixed the drain, it no longer seems to become more prominent when it rains, and it may be drying up & just stained.

We plan to redecorate the room before our baby moves in there, so it's our one chance to get this right (and I really don't want her moving into a room with damp/mould!!!).. I strongly feel like it's something external that's causing this, but don't know what to do to start assessing or remediating. We've had the drain at the front of the property under the ground checked, as mentioned. I don't know if there's someone who can assess the render at the front of the property (it's a painted brick wall, but there's render on the bottom half), to see if that's causing issues.

Third photo (wouldn't let me upload altogether)

You need some overall photos to see what's going on. We often find damp in walls when;

•Gutters overflow
•Roof slates aren't directed correctly into a gutter
•Issues with felt
•Cracks in wall further up

You mention brick higher up and render below. Could water be getting in that junction?

Again, try calling SPAB.

Thanks all - sorry @djh, I missed that recommendation for SPAB, I will try this. I will try have a deeper look at the render too to see if there's any gaps.