Cutting back a rampant blackberry

Hi There, Im a newbie at the allotment game. have just taken over a double plot that is / was massively overgrown. It already had some established fruit bushes such as blackcurrants and gooseberries but also a huge bank of balckberries streching across the whole width of one plot, its getting on for seven feet high and around 20 yards across! It has huge, tall nettles growing through it as well. However, it has fruited massivly and we have a good supply of fruit in the freezer ready for pies, crumbles and jams. My question is how far should i cut this thing back to get it under control again, without damaging the plant and ruining the fruiting for next year, I also need to remove those huge nettles and dig them out. Any advice or guidance would be fabulous.

What I would do is this:

The most important thing is a good pair of gloves! I use the red leather gauntlets intended for welding...

First work out where you want the blackberries to be growing and cut down any that are rooted outside that area, and also the nettles.

 Then there are two options for the ones that are left, either to put up posts and wires and train them (as the books say you should) or let them grow wild and cut them back to their space every so often.

To train them is more work to start with, but makes life easier when picking them. It's not something I've done but I'm sure plenty of people on here have and can advise you better on the technicalities of doing so. It also means you can weed between them! Basically you'd cut down all the old stems which have fruited and tie in the young new ones to fruit next year.

To grow a wild blackberry patch of limited size, cut back old canes which escape from their space and weave new ones back in as they try to spread. 

Either way you will then have to grub out the unwanted stumps and dig out the nettle roots. When bits of blackberry and nettle you missed grow back (there are always some!) either dig them out our spray them. If you have gaps in your blackberry patch you can transplant some of the spare ones. Spraying regrowth from unwanted stumps is safe, they spread above ground by the stems putting out roots where they touch the ground so the unwanted growth is not connected to the roots of the ones you want to keep.

Thanks to you all for your suggestions, I will definately get some thick welding gloves, thats s great idea!
I think I will take it back and try to train it along the original wires that are under there somewhere (i did catch a glimpse of them the other day while i was picking the last of the good berries off).

Thanks again :-)