You don't have to prune it in March if you don't want to, excessive pruning encourages excessive growth in willows particularly. As this one is still small, just take a little off the branches, but I'd be inclined to not prune it this year - it doesn't look particularly overgrown. Never cut beyond the graft point (the nobbly bit at the top of the main stem). If it produces catkins, the time to prune is after they have finished.

Thanks Bamboo - I will follow your advice and not prune it this year.

Having looked again, it looks as if, on the right, there's a branch that's longer than all the others - you might want to even up the shape so it grows evenly this season, lol!

When you do decide to prune it, you can cut it back as hard as you like, all depends really on what size, shape, and how compact you want the plant to be.