What's the latest you can get away with planting out leeks?

My pot of leeks sits there in my cold frame looking at me, but the ground is so wet right now that I don't want to walk all over it to plant them out. How late can I plant them out and still get a decent crop?

I actually had a sunny day - ooh! I needed to plant my leeks out (potted on twice), though I couldn't find my dibber, so used a bit of 2"x 4" and wiggled it around - needs must. I cut off the roots to about 4" according to RHS book which I have never done before, but they went in the holes very well. Despite one of my cats running away with one of them (little tinker), they are all in and watered. I have some trial ones from DTB which are far behind, though sown at the same time, and they were been potted on 2 weeks ago, so they will go in a bit later on. I hope we get leeks throughout winter - 'cos I luv'em!!!

Weather like this is where no-walk beds with narrow paths alongside come into their own.

I should think it depends on the variety ? There are early, late, late-late & over winter leeks. 
Some are March sown, some as late as July, for standing over winter.

On those grounds.....it may be just right, depending on the variety ?  All depends how well the variety "stands" the cold.

Mine haven't gone in yet, I am waiting for the potatoes to mature before I can put them in.  I am hoping they will catch up.  It's a quandary and the only alternative bed I could use is the garlic bed which I am concerned will cause me problems.  So, they will have to stay put until I can plant them on.