Posted: 25/04/2015 at 10:43
It's a bit of both. They're very mature seedlings and very lanky. How old are they?
Even 4C won't damage them. Frost would. Have they lived all their lives inside? If so, they're going to need to be hardened off. If they were mine, and if they'd been hardened off, I'd plant them out now regardless.
If they haven't been hardened off, I'd stick them in the coldframe for both natural light and the beginning of the hardening off process. Then get them into their permanent home ASAP.
20 litre pots? I'd go a bit bigger. And look for depth in the pots to (a) let you bury them as deeply as possible, and (b) provide a stable base for stakes.