I don’t suppose there’s any mad rush Jan as it looks like it’s been like it for awhile, but letting warm air in without any circulation will probably not achieve anything, as it’s the warm air inside meeting the cold masonry which causes the condensation in the first place. If it were me I’d probably put a bowl of salt in the cupboard (which absorbs moisture) and keep the door closed until some ventilation can be arranged, and insulating the back of the cupboard door could even help if there is room to do it. 
I did state there is a crystal damp trap in there. Next time I am out I will look into getting refillable containers.
So you did Jan, perhaps I’d better shut up now… lol
No worries! 
have you ever heard of these
Morning. Well one bloke I spoke to yesterday reckoned it is condensation and that the bay would have been constructed according to proper building regs, including a dpc. I am waiting for his call back next week with an appt.
Easy enough for a builder to check if the is a dpc installed, would take around 5 minutes, and worth doing anyway for piece of mind before any major or unnecessary work is carried out.
Yes, I will get the inspector(s) to check.
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Had someone come round this evening. He will quote for drilling some holes into that wall inside the cupboard, and fitting vents over them. And also to apply a proper paint inside. I will apply the mould cleaner again.
He knew that guy I had before. Didn’t have to say his name. He did! Apparently he is not to be touched with a barge pole and is slightly unscrupulous. Eeeek.
£95. Told him he’s got the job. That includes materials.
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Sorry Jan, only just seen this post I’ve always been a bit on the slow side!
I do so hope it does the job and solves the problem but worth a try for £95, certainly sounds better than £850! 
Anyway it’s what I would do in the first instance. Hope your man does a decent job for you. 
He did, thanks Barry. Gave it a second coat, put the floor vents in and then the carpet. I have put some other stuff in the cupboard now, handy for some dog stuff.