How can you tell if you've damaged a plant beyond repair?

My greenhouse toms are currently purple due to the cold - but they'll bounce back.

Have you got a picture of it?

Potting on wise, you could use a pencil (I use the plant label on small seedlings), if it's not tiny then it will probably grow new shoots. I decapitated one about 6 inches high last year, it grew two side shoots, from lower down- I let one grow into the main stem and removed the other (this was for an indeterminate tomato)

Oh dear- think we've all done it. I think it might be a case of wait and see but from what I can remember it's the defrosting that causes the damage so defrost slowly. I know some plants e.g busy lizzies go mushy and other like potatoes get a brown scorch on their leaves. My feeling is that if they look ok now they probably will be.
As for handling seedlings the general advice is to handle by the seed leaves ( as these will fall off eventually any way) Other than that a table fork under the root ball can help to steady it as you transfer it.

they can tolerate a degree of coldness but it checks their growth, and takes them longer to recover. It's best not to do it but it's not always fatal
It'll live with one seed leaf, it might just take it longer to proice it's first two true leaves [ try saying that when you're squiffy]...
hypothetically that is

A week on, they have not keeled over yet but they are just... not doing anything. I thought it would be the general cold, but weirdly the peppers are fine, they are putting out their 3rd and 4th leaves even though I've not yet separated them into bigger pots. So it got me thinking, what is this? The cold snap has stopped the tomatoes but not the peppers? I thought peppers were the more tender of the two. Or do they prefer to be a bit squeezed together and I was too hasty in moving the tomatoes to bigger pots? (Not huge pots, just the same 4 inch pots I initially sowed them into in groups.) The leaves are a lovely green and not damaged (apart from the ones I mutilated - Mr One Leaf seems ok, the one I decapitated has given up). The stems are slightly red still, but nowhere near as much as they have been. Were they just too small to pot up in the first place?

I'm giving them the best/warmest conditions I can, which is top tier of a blowaway - the only tier that receives direct sunlight in this position because it's an oven otherwise - in the daytime and inside the kitchen at night. I don't have a thermometer but I know it's warmer in the blowaway than outside, and the poor things do need some sunshine. Is it simply not enough? My garden has very slightly colder days (and warmer nights) than the surrounding area.

What are you doing squiffy at that time in the morning Taff?

Sounds like you've been lucky RB. I'd carry on doing what you're doing and keep your fingers crossed.

Rinabean, I don't pot my tomatoes on until they have at least two true leaves and even then they don't go into 4 inch pots. I've just potted mine into 2 and a half inch square pots and they will be left in them till they have filled the pots with roots.

Thank you! I assume the peppers will want the same. I'll be less hasty next time

They're all slowly perking up. Some are now getting their first true leaves (probably all the same variety - I sowed three types and didn't label them because I'm a moron, so god knows which they are) Honestly at this point I'm considering it a success if I grow an actual tomato plant, never mind getting any tomatoes from it! Hopefully I'll manage that at least

Stick some more seeds in if you have some left.