The supers are slow growing, they still don't look too small. Too bad with the weather though. I have 4 plants about that size late start two TS, 1 Bhut, 1 7pod. I will get a harvest about Aug/Sept. I am in zone 7 though. You may have to bring them in at the end of your season if necessary to finish ripening the fruit.

more pics.....

more pics...

Can anyone give any advice on why the leaves are slightly yellowed and on the far left plant, the leaves are slightly split and brown?

too much water. check your soil. mine are doing the same thing. have to be careful when watering.

Although it looks like a nitrogen and/or magnesium deficiency, I'd say it's to much water as well. Your soil looks extremely fine grained. Might want to check out 5:1:1 soil mix from the container forum.

I wouldn't worry too much about the slight yellowing. It is on older leaves only and the new leaves look deep green. Might have been something like a shortage of nitrogen but might just as well be that the leaves are just getting old -- take a close look at any garden plant and you will see old and dying leaves, as long as the overall health is good I wouldn't worry. As for the split and brown leaf, that is clearly some kind of physical damage -- could be a falling branch, wind damage, an animal, just about anything, but nothing to worry about.

Your real issue is that I agree with you that I don't think those have any chance of fruiting outside by the end of a UK summer, so you're going to have to figure out how to bring them inside and give them decent light when the weather turns cool, to let the fruit grow and ripen.