Any idea which type they are? Assuming that's a 10-12 inch pot (for scale) they look a bit leggy as if they're stretching up to get more light.

They are Alicante and I have grown tomatoes in the same place in the greenhouse for many years, with success. This year, for some reason, total failure!

They're a popular type for good reason & you're experienced at growing them, it's hard to tell from a photo so what are you disappointed with to narrow it down?
The list of possible problem causes is pretty big?!

By now the plants should be much taller and bushier, with plenty of flowers. That’s not the case, so something has caused them not to develop as they should. Very frustrating!

Have you changed compost supplier this year? I know a few of my friends, and me, had some compost this year, from same sources as previous years, that turned out to be fairly poor.

I wonder if with all the Eco business & use of more suspect materials if corners have been cut somewhere

Your plant looks a bit short of nutrients, I assume you are following your "normal" feeding routine? Maybe change feed to something else, perhaps a seaweed based one for instance?

My first guess with indoor plants would be red spider mite. Any signs of cobweb type threads? If yes, it'll help to up the soil moisture & general humidity. Stand pots in saucers, trays, etc. & keep them wet

Interesting that you mentioned the compost, because the seedlings got off to a very poor start, took ages to grow to a reasonable size and were generally very poor. I potted them on to different compost and they did improve, but perhaps the damage was already done? I have been using seaweed feed from when the first flowers appeared.

Mine were slow to start but are picking up now, I gave them a couple of doses of Miraclegro, usually I only use Tomarite for toms and cucs, I am very disappointed in my baskets and mangers this year and I also put it down to the compost, I think its rubbish even though its a recommended one that I've used in the past and been pleased with the results...

Thankyou for everyone's comments, very useful.

Using Epsom salts for magnesium might help nutrient uptake

I don't know if you have grown tomatoes in such small pots with some success before. I would agree that the problem, in my book would be the lack of nutrients. The pots are so small that in the hot weather persistent watering has washed the nutrients away as the pots are filled mostly with just roots an little compost. So, next year, much larger pots would be the order of the day thus retaining the nutrients and water. Even at this late in the day, you could re-pot the whole plants into larger pots without disturbing the roots.