The shorting method is worth a try. So shorting the main battery positeve lead to the top (according to Bentley), positive lead should trigger the starter?
I suspect the fault is EWSII based, but I'll definitely give this a shot before I write it off.
I've had that £2000 car for 11 years and did over 150,000 self serviced miles from its 270k ish. :)
My other half's 525i E34 was bought for £750, 5 years ago and apart from needing a fuel pump, is great at 200k miles.
The 2000 E38 740i I have now has needed the buying cost again in parts and 100 hours of my time so far. Great car when I get it right though. I wanted to keep my old E34 on the road but once I'd changed the insurance to my 740, they wanted silly money to insure it short term. In the UK we need to keep a car insured or surrender vehicle tax and declare it as off the road.