As its so close to the fence, you probably don't want to encourage lots of bushy growth lower down, all it will do is grope forward to get away from the fence. Amelanchier often make small trees (especially A. lamarckii), though sometimes they are multi stemmed.

I have Amelanchier lamarchii (canadensis) as a multi-stem which I lightly prune after the berries have finished. There is Amelanchier ballerina, which as the name suggests is a slender, upright variety. As Bamboo says, it looks as though you have bought a tree. If you are unhappy with this tree, then perhaps reduce each branch by one third this year and review the results next season.

Thank you for the comments. It was bought as a tree and is a ballerina. I wanted it to screen the back neighbour’s house wall (hence the Sambucus as well which I love) so if it’s meant to be slender I shall leave it alone. As long as it eventually fills out I shall be happy. I don’t want a Bush there though as I have Gertrude Jekyll and a couple of clems climbing their way up.

Thank you again for the advice.

'Ballerina' has a small tree form - narrow and forms a relatively narrow crown over time, so its growing how it's supposed to grow.