Hi everyone, I just wanted to give an update on this. I think it was a deficiency of some kind, as maybe 3-4 days after I had started this thread, it seemed to stop discoloring quite as much as it was. Ended up giving it soluble tomato feed and seaweed extract, it definitely started to burst into life again.
The absolute depressing part is that it ended up getting really bad budrot, and I didn't catch it on time, probably lost 2/3 of the yield, and had to pull it way too early for fear of getting more mould.
Unfortunately it definitely seems too wet in my country to grow outside very well, especially when it isn't legal. It's such a shame as we had a great summer for July/august which is rare for where I live.
Thank you again to everyone that responded, and though it didn't come out as well as I had hoped, it was a great experience!