Thanks guys!
It's worth re-iterating that I've no idea how I could ever have owned the car without the support of this board - if I had to rely on dealers I'd be broke and on the street .... especially Wout's massive thread on his rebuild!
I took my time considering what to do, then a long chat with Davioli helped me make my mind up - there comes a point when you have to stop spending money on something and I think I reached that point - if the rust was relatively simple to fix I would have kept it, but it's not really.
My original plan to transfer the mechanicals to Alpine Amy stalled as I realised that by swapping engine / suspension / etc. all over then I would want to renew every linkage, bush, arm, mount, etc. and it would end up costing £3K in parts and I just don't want to spend that money on it at the moment.
I'll hang around anyway to give the benefits of my experience to other owners - I learned a lot about the car - and also because it's a nice forum and I love M5s just as much, so want to keep track of how you guys are getting on - I have a good feeling I'll own another one in a couple of years ..... once I get over my Hondas and sell them I want a GT3RS (don't we all?

) and an M5 for comfort
Pics of the Hondas here if anyone is interested ....
BTCC-spec one - no bumpstops and fully rose-jointed, no use for daily driving - crazy short ratio gearbox but standard engine
Picasa Web Albums - nebpor - raceTegra
Road (i.e. track!!) spec one - this one is getting an interior put back in and less hardcore suspension, so I can use it daily - had hardcore 10,000rpm engine build but has went back to standard as the high compression just kept wrecking it ;-)
Picasa Web Albums - nebpor - ITR#