Update:
I am now the owner of my first Z4. Picked it up yesterday at High Wycombe before immediately driving 300 miles north yesterday and another 200 miles further north today.
First impressions:
It's a solid old beast. The 2016 MX-5 I traded in for it felt like a bag of feathers in comparison. This thing brutes its way everywhere although handles surprisingly well for the extra half ton it weighs.
The engine is fantastic, I've had 4 pots for the last few years and this blows them out of the water completely, really have to keep an eye on the speedo. Had a VX220 turbo a few years ago and it gets to 60 in the same amount of time but runs out of puff at 70 - the Z just keeps on pushing.
It's equipped with runflats so they are going in the bin soon, and the bumper has a scratch that the body shop found which is being restated at my convenience as part of the deal. Very happy I made the jump, will stick up a new car thread once I've put a few more miles on it (provided I don't bin it in the Highlands). It has a few minor marks but it's an 8 year old car, can't expect perfection.
Thanks all for answering my queries, it was appreciated.