Update time....
Had the seat in for 2 weeks. Love the look, far nicer than standard. On a short trip, the additional support from the inflatable bolsters is excellent - completely solves the issue of sliding around when driving enthusiastically.
On a long journey however, they're too uncomfortable - if I'm completely honest, that's because I'm 6 feet 2 inches and chunky with it

and the bolsters, in their lowest state of deflation, are just too tight. If I shed some chunk, I'd slip further in between them and I think they'd be perfect but it's a backward step comfort wise at the moment. I'd like to try the Z4 M-sport seats for comparison but I'll probably find the same result.
There's a 2nd minor reason for the U-turn too. I have a dodgy right knee and due to my height, the only way of entering and exiting the car without causing pain is how a skirt wearing lady would i.e. arse first, then swing both legs around into the car (as opposed to left leg first, then arse down on seat, then right leg to follow). In the standard seats this is fine, but in the M3 seats there is a pronounced ridge on the right hand side of the seat base - so I think this would get worn out very quickly with my weight crunching and swiveling around on it all the time.
So, removed the seat today and re-installed my original drivers seat - all back to normal, except for the airbag light I have triggered which remains on.
For reference, and for any future thread readers: The cheap and cheerful B800 tool does not work on the E85 Z4 - at least on mine, bearing in mind mine is an 03 reg and the tool is advertised to work with vehicles 'up to 2003'. Doesn't matter, it was worth a £11.50 gamble (it plugs in, powers up, but detects no faults).
Carly BMW app: identifies what I believe to be the fault (attached), attempts to clear all outstanding faults on the car and reports success, but the light is still on.
So, I'm assuming at this point all is fine on the car and I just have to get this light turned off, rather than there being a persistent fault which needs sorting before Carly will clear it - so INPA lead here we come.
Thanks again anyone who chipped in on this thread - seats will be in the for sale section shortly.