The silver part stamped ATE is a hydraulic /Power Brake Booster. It looks similar to part number 34331156925 that was used on quite a few BMWs, have a couple of links:
Have a read of this, its actual going from what you have to a servo but has lots of info.
Keep us up to date if you do discover anything as I don't think I have seen or heard of it in an E30.
Thanks for the info. It looks like quite a complicated set up! I think I will stick to the clio servo so if any of you guys are interested in taking these bits off my hands, drop me a message. I seem to have all the components shown in that diagram but will check today.