UPDATE - Problem sorted...
Did some reading up on the boxer twin engines and took some mechanical advice and there's a huge difference between replacing a cylinder on these engines compared to messing with the main engine block (short engine?). If it had been an issue with the engine block, then I don't think I would have contemplated a repair...but it wasn't that so didn't need to go there.
So, decided to give the new cylinder a go and the fix appears to have worked. Dealer did a 40+ mile test run, and I've done a further 60+ miles on it, and no problems as of yet - fingers crossed it stays that way.
It was a manufacturing fault with the cylinder that unfortunately didn't become evident during the PDI, and during my initial inspection of the bike. Turns out there was some extraneous metal on the flat cylinder surface that mates with the flat surface on the engine block, preventing a pucker seal. Over to BMW & suppliers to up their QC...
I couldn't be more complimentary towards SPC in the way that this matter was handled, even though it wasn't their fault at all. They cleared the decks to get this sorted and got a loan bike out as soon as practically possible (I didn't need one on the w/e - did something else instead).
Was totally stoked to have Bob come up to me and say he'd managed to wangle a pair of BMW Ali panniers FOC as a compensation gesture. Luvli.
So, on the basis that the bike manages stays in control of it's bladder, issue closed.