Thank you everyone for your ideas and comments. Really appreciated, and many commiserations to others with the same issue.
I don't believe this will burn out until the cause is identified.
I have looked at possible antigens - coffee, wine, chocolate, some fabrics, washing powder but nothing stands out. I also thought about heat rash. After I lost a ton of weight I found I was feeling very cold. I put on a few extra "layers" and ended up getting too hot! I found that applying a cold sponge to the (many) lesions helped cool them down and "dowse" the itch at least for a while. I also thought about doing some allergy tests from online suppliers, but I don't trust them at all, and suspect that the results would be very non-specific.
I have used body washes (QV), menthol cream and simple soap but to no avail. Hydrocortisone cream (1%) and Eumovate ( help very little. Antihistamines help also very little (tried cetirizine and currently fexafenidine) but again tbh no great results.
I started taking calciferol (already on B12) which seemed like a good idea.
After doing a bit of research, I came to the conclusion that the cause of the irritation might in fact be fungal (candidiasis) because the itch and rash have spread to front (through constant scratching!). So GP put me on an oral anti-fungal (Terbinifine) yesterday. Very early days but there just might be a tiny improvement. Couldn't use fluconazole because I am on clopidogrel and contra-indicated.
This is a very debilitating afflction! Good luck to everyone who is trying to get rid of this, and if anyone has successfully dealt with this, pray tell!!