UPDATE TIME! Bet you've all been waiting for this one.
Think I've managed to solve this problem, albeit in a rather Heath Robinson fashion, and thought I'd share, for anyone who wishes to replicate this level of anal retention. First off, thanks to those who mentioned Brodit. At the time, although they didn't offer a GR86 mount (predictably, they started selling one for the GR86 about 2 days after I ordered the phone holder - however, their mount fits over the air vent, and the whole point of this thread was that I wanted something tucked away and not super obvious) but I started to look at the phone holders themselves, and ended up ordering this -
https://www.brodit.co.uk/brodit/holders/_/_/521843.htm
It's an adjustable holder with built in USB. It's a very simple, but well built design. There are two screws on the face which you slacken off, and this allows the sides to be moved in or out, depending on how wide your phone is. The USB plug is also rigid, but has a similar screw allowing for adjustment. So, loosen the screws, drop your phone into it, get the size right, and then tighten the screws - bingo, the phone now fits the holder perfectly. I should note, they do make fixed holders for nearly every handset; however, as I discovered after ordering, they are completely fixed and have no allowance for phone cases, and my phone has a bumper case which I can't do without really. Luckily, I'd only removed it from the packet before realising, so was able to return it for a refund and order the adjustable version.
The idea is that you then attach the phone holder to the mounting plate that fits your vehicle, using the ball mount on the back - it has a flat plate with 4 screw holes....
As I wasn't bothered about having the phone mounted in my eyeline, I had planned to see if I could remove this ball mount and just stick the bit holding the phone to the dash. Unfortunately, to do that means taking out the two set screws which allow adjustability, and that means it doesn't hold together very well. This means leaving the ball mount in place and instead attempting to stick the holder to the dash using trim tape attached to the flat surface which is designed to be screwed to the Brodit vehicle mount. I didn't think this would work - it takes a far amount of force to push the phone home into the USB socket, and I thought the additional bulk of that mount would mean it would just be levered off wherever it was stuck to. But figured I'd give it a try.
To the car! [*transition*]
In the car, I was looking for somewhere I could mount the phone. Space is limited, but as I said, I don't really need the phone somewhere I can see it - just somewhere I can place/retrieve it from easily. I was looking around on the centre console next to the gear lever, but settled on placing it on the cowling next to the steering wheel, so that while in the holder, the top of the phone is next to the start button. I cut two pieces of extra strong number plate tape, put them on the back of the mounting plate, and just stuck it directly to the trim. So far, it seems to be holding securely!
Then it was just a case of tucking the wire up behind the knee pad, and then running it along the centre console tucked behind the trim, and then up and over into arm rest where the USB sockets are. This is the only bit I don't like at the moment - there's no way into the armrest except for going through the top. It would be nice if there was a way to run the cable in from the bottom so it's completely hidden, but doesn't look like that will be possible. without drilling holes and I'm not prepared to do that.
And hey presto! All done. I've given it a couple of test runs, and so far, the adhesive tape is holding with enough force that I can simply slide the phone into the holder and press down, without having to use a second hand. And the same for when withdrawing it. The phone is mounted down out of the way and out of sight, but being right next to the start/stop button, I'm unlikely to forget about it when exiting the car. In my driving position, it's not in the way at all.
I took some pictures with my SLR to show the context of how it looks with the phone in place. I'm no David Bailey with a proper camera, especially on dark interior shots, so excuse the quality, but you get the idea.
(I can be a picky t**t too, don't worry)
