Not until all risk of frost has passed......probably mid-late May..

Not for another couple of months (when all chance of frost is gone) really.

You'll probably have to pot them on into something a little bigger, but whilst you want to maintain a healthy plant to put into the garden, you're going to want to contain it too. If you have a green house (unheated), put them in there at the beginning of April & hope that they'll be ok - they should be barring a return to the artic conditions of January.
I can't believe they're selling them this early.

Also, when I put mine in, I dig the hole a good 18 inches down, mix shredded newspaper in with the soil before putting it back in the hole & planting my courgette plant, Water it in well, make sure that it is well water during the height of summer when it's bearing fruits. Straw laid around the plant is a good idea too to help prevent rot.

thanks amblealice and meanie. a spot of tlc on the meter box in the porch for ten weeks then