Hello chaps, can't quite believe it's been almost half a year since I posted on this thread; don't worry - still got the car and still working on it! In late July I also bought a Land Rover Discovery 3 tdv6 - which in the 6 months since I bought it in July needed new rear discs/pads/shoes, brake hoses, 2 front wheel bearings, full cambelt service and boots on NSF driveshaft... It's been up on ramps more than the e38 recently:

That said, it's a great car now that the bits left wanting at around 100k have been sorted.
Since my last update, I've made a few bits of progress worthy of note.
Next job was to install the filler. I knew from the start that I wanted a location on the bumper, rather than a hole in one of the wings. I am aware of the COP11 amendment suggesting a minimum height of 500mm so as not to be awkward. As it happens, the filler is in the bumper, readily accessible at 450mm height, on the offside - same as the petrol filler.
Tinley tech supplied a daunting looking core drill bit, and I bit the bullet. No pictures of the drilling in progress (too nervous to stop!) - but I went through the bumper and the plastic reinforcing support behind it - at a point where the support wouldn't be weakened and I could still install the necessary boot on the rear of the filler. Here it is installed:

Pipe work routed through the rear arch, and then on above the rear subframe:

At the closest point to the exhaust at the back of the filler assembly, there is, just, 250mm clearance to the rear of the exhaust back box, which is covered by the vehicles existing heatshied, what the picture doesn't show so well is that it's also in a corner of the bumper assembly and you couldn't get to the exhaust if travelling directly from the rear of the filler assembly. There's certainly enough distance, in a direct measurement to the exhaust tips at the rear of the car, enough that I haven't botherd to measure. I'll use some heat shielding on points of the filler and tank feed lines that travel closer to the exhaust around the rear subframe, where it isn't protected by the car's existing heat shielding.
At the moment, the boot on the filler is pulled back so I can get to the gear behind the filler, and the wheel arch cover isn't back in as I still need to do some wiring - but all of that will get done before it goes for testing. I should say that any pictures of the wheel arches, where you can clearly see the tank or filler lines aren't representative of the final product - of course both the factory arch liners will go back in when the car's finished.
Here's the filler line as it travels under wheel well into the tank area - some more edge stripping added to the vehicle heatshield to stop chafing:

What I do need to do is make up is some spreader plates for the tank mounting bolts - much bigger/wider than the standard washers. It was brought to my attention that the spare wheel turret is in fact only screwed/bonded in to the wheel well - so it's not a permanent part, meaning I need to spread the load around the bolts between the turret and the permanent bit of the well. That can wait till I'm a little further down the line though.