This is the money quote - "This doctor suggested I have a patch test. If you haven't seen these before you basically have loads of different potential allergen

This is the money quote - "This doctor suggested I have a patch test. If you haven't seen these before you basically have loads of different potential allergens stuck to your back on a Monday (including a range of chemicals including stuff like nickel), on Wednesday they are all removed and then you go back on Friday to see which bits of your skin have had an allergic reaction." Working out it was the spray in the toilets was easy once he knew what to look for.