Well in the end when MM works, it does just that!
So after Wednesday's dismal performance of visiting my 2 closest Asda stores with Smart Drop Kiosks, only for both of them to be in some way out of order, I went back to one today. It was working and the whole process was done in not much over 5 minutes and the full quoted price was paid (to be fair if MM had a 'As New' category it could have passed for that rather than just their 'good' as there wasn't a mark on it, even hairline from what i could see). £800 total for a 512gb 13PM to show that re-saving quotes works nicely

.
I saved a quote with them on the 24th Aug (Far Out event announcement day) for £780 + a £10 voucher. Added it to my basket 13 days later (didn't want to risk some sort of mid way through the day cut off on the 14th day), removed the £10 voucher and replaced it with 'UPGARDE22' for the £20 bonus, but the just re-saved rather than proceeding to checkout. Left it there for another 13 days until Monday when i proceeded through the checkout, and on what, day 5 of my 21 days to get it to them, the Kiosk worked a treat. After the pain shared on here re BACS over the weekend i went for PayPal and the money was in my account 5-10 minutes later. Just checked the offer price on MM today and its down to £612 from the £780! (That said Rebuy look to still be offering £758 for an 'As New' phone, but I'm guessing much more of a chance of someone else not seeing it as that). And even on eBay the realistic going rate only looks to be £850-900 (minus fees and postage) ish from recent sales, I'll sell the box etc on there to recoup some of the difference, but happy to lose a bit of money to avoid the jeopardy that inherently comes with eBay (not that I've ever had an issue, but there's always a first time).
So anyway, lets hope that trick still works next year (probably shouldn't have posted this

).