What on earth to do with pea seedlings now?

I'd be really grateful for some advice on what to do with these peas, please! I've got three trays of peas growing: the one foremost in the picture are Kelvedon Wonder, the ones in the middle are sweet peas, and the ones furthest away are Douce Provence. Which had a really bad germination rate, incidentally: only 12 out of 32 actually grew!

I know Douce Provence are early peas and Kelvedon Wonder are later ones, but as I had some Kelvedon Wonder anyway in my seed box I thought I might as well just sow them also. Not sure why the growth rate is so different, considering both varieties were sown and grown under exactly the same conditions mind you.

Anyway, I'm just thinking what to do now with the plants that have grown ... I'm wondering if the Kelvedon Wonder peas are tall enough that they should come out of those root trainer pots. I've got a seven foot long strip of earth dug over that I was planning to put them in, so they'll grow up the shed wall, and about a month ago I dug half a bag of "well-rotted farmyard manure" into it. The soil is clay and was dug over from grass a few weeks ago, so I haven't used it before. I've also got another strip of ground against the wall of the house for sweet peas, where I grew them last year.

The earth outside is rock solid as we have frost at the moment but never having done this before, I'm not sure what to do now! Do I leave the seedlings where they are or will they keep growing over the winter? If I have to transplant them out now, is it just a matter of hacking at frozen ground and trying to make holes in it to sink the peas in or ..? And given that the weather here is horrible and it could be a hard winter, I'm not sure what to do regarding covering these peas. Someone at work suggested putting plastic milk flagons over each individual plant to protect them over winter - not sure if this is a good idea or not? And what should be done re hardening off?

Also, will these seedlings get much bigger over winter, or will their growth be pretty much checked now?

Any advice on what the heck I should do with these peas would be so gratefully received, thank you!