I would wait until the middle of April when frosts should be over. Make sure to only plant at the same depth as they don't like to be planted too deep. I wouldn't feed until after flowering, and then with someting like Q4 but sparingly.

As it is in a pot and I assume it has been outside it can go in any time now. We still get frosts at the end of May so it is better in the ground now.
I have planted out 2 in the last week. The rain is helping them settle in.

I agree with Jimmy about them not being too deep.

Thank to you both. It is in a pot but have kept it in cold frame over winter so will wait till after risk of frost has gone.