Got the car back. Not convinced that it is any better. Will find out if the fault is back after a few times through the Tyne tunnel on my commute next week. Will give it a blast later today to see if it is any more sensitive to being close to the car in front with the <car>!<car> warning. My black spoiler trims on the boot lid are a little straighter (although still a gap at the top on one end, but the outer edge follows the edge of the rear window perfectly so I think they are cut wrong in the first place (like an incorrect angle on a piece of skirting board or coving!).
My Blackvue 650 kit has arrived from Amazon.fr, approximately £52 cheaper than I could get one here in the UK for (with 64Gb card and magic power kit all in for £267 delivered), but i'm sure i'll be too pissed to fit it tomorrow and too hung over on Sunday after my cousin's 50th celebrations tomorrow. I should wait anyway - poor planning on my part as I forgot to order one of those cheap trim removal kits from ebay on the day I ordered the Blackvue kit. It is on its way. A job for next weekend I think.