Tomatoes for the windowsill

Something bad is happening to my windowsill tomatoes. After I watered them yesterday, one just keeled over. I managed to get it propped up again, supported by the other plants, but the stem is part broken. I hoped it was still going to be able to move water up from the roots but it seems not as it's completely wilted today. I tried seeing if I could pot it deeper so the broken stem was underground, but I don't have any containers deep enough for that even after removing most of the existing compost from the root ball. So, it looks like I'm losing this plant and all the tomatoes that were just waiting to ripen.

Now I see that two or three other plants also look a bit floppy and I don't know why. I have watered them quite a lot this last week, but it's been extraordinarily hot and they are in very small containers and have seemed to dry out between waterings. I may also have given them tomato feed (once per week at most) that is a touch too concentrated as I am watering using a 750ml bottle and the measuring cap is not sensitive enough for such small volumes of water. Otherwise, I don't think they have been subjected to any mistreatment.

I feel incredibly guilty about killing one of these lovely plants and will be really gutted if I lose any more, but I don't know what to do differently. Now the weather is a bit cooler, I can cut back on the watering a bit, and I don't need to feed them again. But if it gets really hot again, they are going to need daily watering or they will die anyway.

My poor little tomatoes ☹️☹️☹️