how to prune blackberry bushes?

this is a bit of a strange one with factors involved that are out of my control, so not expecting a text book answer.

what i need to know is how to prune blackberry bushes?

the trouble is they aren't actually my bushes, they are growing against the other side of my back fence (6 ft ). they are in between a large industrial unit and my fence on waste land/ brocken concrete/ rubbish. i can't actually see the bottom of them so can't see what they're growing in, so obviously any feeding or mulching is definitely out.

so basically i can get to half of the bushes from 5ft high, up to the top. what if anything should i be cutting back?

there's still the branches from last year with the bits that produced the fruit (not very scientific but i'm sure you know what i'm talking about).

thanks all.

Blackberries crop on 1 year old wood. So tie in last years new growth and cut out the old wood.

Can you put in some bamboo canes or similar to tie them to? It will make picking easier and also stop them rooting at the tips when they touch the ground and getting too near your fence.