A quick visit to the plot today showed the snow is still covering the ground, but it is melting. I looked at the water butts and the ice in them was melting too. There was a big plug of ice in the middle of each butt which was stuck to the lid, but water was all around each plug. When I checked the tap in the greenhouse there was running water, so the pipes seem to be intact.
So no leaks in the pipes, but leeks in the ground are certainly still there. They are exposed now and look fine. The winter onions and garlic are exposed too and they look a bit limp and yellow, but I expect they will pick up. The days are getting noticeably longer so the extra sunlight should help them.
All seems well.