It'll be okay in a pot, if you pot on as necessary every year or couple of years, but there will come a point a few years down the line where it won't do well because its restricted at the root, and supplying a larger pot might not be possible.

Best potting soil to use is John Innes No. 3, or a mix of that and multi purpose compost. John Innes, whilst its the king of potting soils, is pretty heavy when damp, much heavier than multi purpose, but its also much better for permanent, woody planting in pots.

Thanks Bamboo, I was going to put it into a 55litre pot, as I dont have a garden at the back, so everything is going in pots...would that be ok to do now so it can settle into its new home, your comment above is very helpful

welcome to GoY.
yes I agree with Bamboo. chose a pot that has a wide neck and doesn't narrow below it or you wont get the plant out except by breaking the pot. I made that mistake with a lovely terracotta ali baba pot.

I'm trying to visualise a 55 litre pot, with little success. Note that it should be planted into a pot that is not more than 2-3 times larger all round than the rootball of the Salix - when it fills up that pot, move into a larger one, using the same rule of thumb for sizing.

If your 55litre post is a lot bigger than the pot its in now you could pot it now into a larger pot than the one its in now and then put that into your big one, backfill the big one with compost and plant bedding plants in the space between the Salix's pot and the big one. You can easily lift out the Salix and pot it on as necessary until it outgrows the 55l pot.

Hmm, not sure that's a great idea, Steragram - there's a high risk of the Salix rooting through the holes in the bottom of the smaller pot into the compost round about.

That's true Bamboo...it would need checking every year.