It will have made a lot of growth this year and its roots will be quite established, so i would say that it would be fine to cut it back in the early spring - it will have died back a bit anyway so you can see where you'd like to cut it back to.

I have two here and one i cut back to about 3' and the other i leave alone.

Thank you.
Is it like the Elderberry we see growing in the countryside, just a different colour? I know the local farmers cut back heir hedgerows at certain times of the year with no ill affects to the shrubs. I would like to "spread it out a bit" without it getting out of hand, or should I get another one or two, to get the same effect?

It 'is' the same genus.
You can hack this plant about and it'll just keep coming back, it's tough and you can't do it any lasting damage, in that respect.
To make it spread out and across you could keep pruning the top and it should make growth from the sides, if you wanted a denser look you could plant a couple together i suppose.

The two i have are in different parts of the back garden.

Be careful what you wish for though! I pruned the top of mine...in a large pot...and its spread horizontally is amazing! I'm now wondering if I should prune it in Autumn as its branches are obstructing the path! ...or should I wait until Spring? I keep threatening to put it in the ground but I like to see it from the patio window.

Louise & Izzy: thank you for that information, very useful. Based on that, I shall crop the top & see how things go before having a second shrub, if I have a second that is.