Just one other thing I had very similar on my ground floor which I believe was a stable origionaly now our spare bedroom and second sitting room. I got sick of scraping and painting that bit every trip. I got round it by putting batons on the wall and tongue and groove wood cladding which I then painted. A very neat tidy job when finished with an air gap between cladding and wall with the cladding being breathable. My problem was solved. I got the cladding Barton's etc from Leroy merlin not cost prohibitive by any means and easy DIY task. I think I have some photos I will post if I can find them.

Thank you all so much for helpful comments. I will try all suggestions and any more most welcome and appreciated. Im baffled by the fluffy white mould unlike the usual black we normally get with damp at home but will leavewell ventilated in future when locking up 'bobd22'.There are no pipes in this wall and as an internal 2 foot thick supporting wall its weird that this is the worse area 'snikpoh" new inner wall sound like alot of work . . I teresting comment about sand reaction with damp 'hey jude'. Im off to google "nitre'. Please continue with your great advice and thanks to all.

Hi. Not sure if my previous message appeared as I cant seem to find it but thanks to all advice and help. I will definitely try changing ventilation routine cheers bobd22. Interesting comment on 'Nitre' thanks 'Hey Jude' so googling that. There are no pipes Snikpoh and it is a supporting wall in the centre of the house although Rosemary has just made a point about possible leakage from above. The roof is staggered directly above this wall so will investigate that too although there is no problems upstairs. I did have the rear tiles removed and re-laid so will make sure they are ok along that edge. Bathroom is defo coming up through the concrete though so will investigate that further. Any more comments, help or advice much appreciated. Than you

It is very strange how this phenomina affects walls, for instance my lower exterior wall was the least affected the worst affected one similar to yours was an inner middle wall, exactly the same symptoms you describe. It bugs us Brits initially as not something we are used to other very common thing is tiling it? I preferred the cladding route as breathable?but I suppose time will tell so far so good around 3 years now! Someone also mentioned the salts white furry stiff on tiles on lower floors. I get that in spare bedroom very slightly but no where near like I used to pre ventilation and not at all when occupied.

Yes bobd22, very odd. funny thing is when I bought the house it had cladding up the wall covering this area but I removed it as it was a bit worse for wear with no idea what was behind . I may re-do it now. Thanks

Yes I hadn't noticed too much when we bought and when trying to tidy up noticed while scraping walls this specific area had what looked like plastic coating which peeled off, then the fun began.Just out of interest another way that I know people have resolved it is to use a combination of Caucho Paint together with Fibreglass. A friend did this and it is still good 7 years later. Seemingly you clean off the walls getting rid of all the rubbish. Apply Caucho then while wet apply fairly fine Fibre glass let that dry then apply caucho over the top again.That way if you dont like the cladding route you end up with a normal looking wall. Caucho is easily got in the Feritaria's as is fibreglass by the role known as " Fibre" as in Fib re . Go for the finer fibre which leaves a nice smooth finish. By the way why it didnt work long term on our house was that whoever had put the caucho on didn't use the fibreglass so it wasn't strong enough to prevent being pushed off by the mould. As I say our friends after 7 years is only just starting to show a need for some repair, their wall by the way was not just mould but actually wet.

Brilliant thanks. will have a look for it when I visit in September.

from wikipedia;
Niter/Nitre
Niter or nitre is the mineral form of potassium nitrate, KNO₃, also known as saltpeter in America or saltpetre in other English speaking countries.
Niter, potassium nitrate, is found as an efflorescence in hot, dry regions.
Explore: Potassium nitrate

Nitratine (also nitratite), also known as cubic niter (UK: nitre), soda niter or Chile saltpeter (UK: saltpetre), is a mineral, the naturally occurring form of sodium nitrate, NaNO . Wikipedia
Explore: Nitratine, Sodium nitrate

Niter was used to refer specifically to nitrated salts known as various types of saltpeter . . . by the time niter and its derivative nitric acid were first used to name the element nitrogen, in 1790. Wikipedia
Explore: Potassium nitrate, Nitric acid

That is damp, penetrating rising or a leaking pipe. I have seen much worse. If it was a leak I would expect it to be much worse.

Painting the exterior will help reduce penetrating damp.

Keep the house aired :
Put in an air brick and/or leave two window slightly ajar to promote circulation through the property.

Thanks missile. will definitely be airing as much as I can but I only visit a few times a year. Defo not a pipe as an inside wall and the bathroom and kitchen are in another building separate from the house. Will look into getting an airbrick or two but the walls are so thick (2')! Thanks for advice and comments

I had a similar issue, although my walls were not 2ft thick!!!!

I got some paint from the fereteria. It was clear and acted as a barrier to prevent water penetrating. Sorry, I can't remember the name.

Good luck