Your asking for a diagnosis from people without anyone seeing the job which Is impossible!
It sounds just like what most of the guys have already gave you an answer with it being condensation.
To me this is what it sounds like.
You need to install a PIV or HRC depending on your circumstances but it would be best having a company to assess your individual requirments.
If you want the cheapest option then these can be bought online and installed yourself but need to be done properly or they won't be any benefit at all.
As its not a living space then I wouldn't bother tanking, battening, boarding etc but use your money and invest it in a unit!
That KA tanking is not worth the bucket it's supplied in!
Also the preserver you used wouldn't work as the wood was already infected when you used it.
If I'm correct the preserver you bought is for use on timber before infection as it will stop it.
You need something that will kill the infection then to protect in the future.
Also the reason your walls are wet behind the membrane is because there effectively sweating out!
Don't cover them with any sort of b an q membrane as this will make it worse!
Damp proof membranes are for under concrete slabs not on walls.
This is where a cavity drain membrane is used.
They have a egg box style mould which allows the walls to breathe eliminating sweating and in most cases allows the moisture to evaporate behind of if its running water then guides it to the drainage which it is usally installed with!
most people on here know what there on about or at least have some ideal so when you ask for advise and a majority answer is given at least take it or say thanks but ill leave it for now and not try and get round the problem as cheap as possible and then ask for more advice when its not working!
Hope this helps in some way!