I've been further assessing the health of the tomatoes this morning and it doesn't look good.
Early blight? Septoria leaf spot? I've looked at pictures and descriptions of both and it doesn't quite fit the bill for either, plus they usually strike when the plant is setting fruit, and these are way off! You can't really see it in the picture, but some of the leaf tips are tinged very blue as well. If I'm lucky it's just a deficiency of some kind and they'll recover when I plant them out in the big wide soil, but I won't get my hopes up. Around half the plants are affected, with a few not-sures, so I've separated them out so that it doesn't spread any further.
So now I'm wondering if I should make a late sowing to replace these plants if they don't make it. By my calculations we've got 131 days until the end of September, so I reckon there's time. And if I've infected the soil by then I could plant them in growbags instead.
I've got another gripe with my tomato seedlings too; they're still tiny! I suspected it was because I hadn't potted them up to bigger containers and they needed more rootspace, but when I received my tumbling tomato plants in the post from Suttons they were big healthy plants but with rootballs a fraction of the size of mine!
So where did I go wrong?!