I would just let them alone. If you cut them back, the bush will have to expend more energy to regrow more canes for next year at the expense of black berries.

Hi Hank, you can cut them back when they are between 5 - 6 foot tall but not before; remember to remove all he canes that have fruited this year.

I would leave them longer than that if you have room. It looks to be doing stupendously well this year! If you train them all to the same side it makes cutting off the old briars much easier.

I'd still prune back to 5 - 6 foot... quite long enough in my view :)

Are you quite sure M ? But I want these new canes to grow all along the wires on the fence as they have this year - they're about 3 metres in each direction on 2 wires at the moment, and I was going to fix another wire a foot above these for an extra layer.

That was my point Hank. Blackberries fruit on canes that grew the previous year. You wouldn't want to cut them back entirely, but only by 1/3 over the winter while the bushes are dormant. This will encourage branching and more blackberries. Once you harvest the berries, remove the canes entirely to make way for next year's canes.

Hank, if you have 3 metre wires to run them on no problem. Just do as Bathgate suggests and cut back by 1/3 over winter.

Thanks all, I now know exactly what to do. Am looking forward to lots of blackberry and apple crumbles again this year