What do to with this mess?

I've got a bush in a similar shaded, north facing, position. Its a Merton thornless blackberry, and its about 6 years old.  It fruits really well (on the second year wood).

After it has finished fruiting (about now!) I cut all the old wood shoots that have fruited back to about 2" above the ground. This will leave several shoots that have grown this year and which will fruit next year. I choose 3 of these shoots to keep, and again cut back any others to ground level. I trim these 3 shoots back to about 5 to 6 ft above the ground, and tie them into the trellis. Finally I spread a couple of buckets of compost as a mulch, but make sure the mulch doesn't touch the bush.

Next year, give it a bit of high potash fertiliser when it flowers, and water the roots if it gets very dry.

Last thing. I don't know if this applies to all cultivated blackberries or just Merton thornless. When the berries appear, they look black and ripe but are still tart and bitter. They are only fully ripe when a berry has just lost its shiny gloss and is slightly dull.

Hope this helps. It works for me!

I had a thornless BB which wasn't producing well.  I was advised to cut the old branches (those which had fruited) back to the ground.  I was then told to tie the remaining branches onto the supports in an upside down U shape with the tip down towards the ground as this reduces the energy that is put into growing the branches and leaves it available to the plant to put into fruit production.  Worked a treat for me this year, I got tons of BB's and was able to make jam for the first time.