I think I have an explanation for the return and growth of the splodge on my chest. Over the last few weeks I've spent some time in the company of a vegetarian. I have nothing against vegetarians. But meals tend to get produced which appear to be chicken but are in fact made with the Qorn meat substitute.
What is Qorn? It's a fungus, like mushroom. So this tends to confirm the theory that I am reacting to fungi. Originally the dietitian said mushrooms were problematic and it depended where they were grown. I assumed this meant that if they were grown in salicylate rich compost - as most commercial mushrooms would be - they were off limits. On this basis Qorn should be okay as, according to Wikipedia, it's grown in vats, not in compost, and fed merely with glucose, vitamins and minerals. But it's obviously not okay.
Thanks to a small amount of sunshine over the weekend the splodge is fading a little and is less red and itchy. Let's hope this continues. Meanwhile I have told everyone, sorry, I won't be eating Qorn again.