Survey - second opinion

[AzureBiscuit]

I am buying a Victorian basement flat in London. One wall in the flat has a very severe mould growth. This is an outer, north facing building wall. My builder suspected none/ineffective DPC and / or DPM. He suggested rising and penetrating damp.
A surveyor came and concluded all damp is from condensation and there was no rising or penetrating damp.
Once I cleaned the wall and dried it somewhat (gypsum plaster exposed), I noticed wet patches near the skirting board after a strong rain. The patches wouldn't have been visible before because of severe, dark mould.
I raised with surveyor saying he seems have missed water ingress from outside (be it rising or penetrating). He is not very responsive, but insist he will need to revisit the place before he can comment. I am not around to meet him. And the wall was painted nicely with an ant mould and anti damp paint, so no patches whatever to see any longer.
I think to ask a second surveyor for opinion, presenting the known facts. I believe the first surveyor was wrong about condensation as source of damp.
Could anyone recommend a reputable surveyor, ideally expert in damp problems and based in SW London?
Thanks

Have you already bought the flat?
It does not matter how well a basement is ‘damp proofed’ it will ALWAYS be damp. They weren’t built for human habitation. In other countries they’ve converted basements into garages. Unfortunately in the uk it seems we think it’s acceptable for humans to live in them. It isn’t.

Yes, agree, it's only in the UK, the only European country I have seen people housing in basements.
Sadly I exchanged contracts and paid drown 10% already, for the large garden and bit more space than an average flat, while accepting subterranean level. Now scared of water penetrating constantly and even thinking walking away and leaving the 10% to the seller :(

Why did you exchange on a property where you are still engaged in survey conversations? Normally, you would finalise all that and then agree to exchange. I'm confused as to how you even got to this point.

First survey stated just condensation as source of damp, hence I paid the deposit and was allowed to start work before completion which was scheduled for a later date. During works , the wall was cleaned and wet patches after rain were noticed.
My choices now: get a second survey and if it's very bad, walk away forfeiting deposit but saving 90% plus potentially huge bill for damp works. Or do nothing, pay the remaining 90% and hope for the best.
Can anyone recommend a good damp surveyor? I don't trust the first surveyor's conclusion any more.
Thanks
Edited for typos

I would walk away. Living in a damp basement would make me miserable.

Will it cost less than 10% to fix the damp issue? Worst case scenario is fully tanking the basement, so installing a drainage system and pump and them putting membrane on the walls and building secondary walls internal to those with drainage in the cavity in-between to take any water which comes through away.

Good point, thanks. Need to get those quotes, sounds like a lot of work going into 10-30-30k maybe...

Ah right - understood now. I agree with PP. It comes down to the maths. Get quotes and then make a decision.