Managed to find the right pre-2013 front seats on eBay to replace the incorrect fit ones [
https://beemerlab.blogspot.com/2017/05/f10-530d-black-leather-rear-seats-and.html]. They were for sale on their own, the owner having swapped his front seats for M-sports and was willing to let the old ones go for just £140. They were from a 2011 car with 80k miles and in very good nick, only snag was they were in Northampton, so I removed the passenger / rear seats and did the 6 hour round-trip to collect them. They bolt straight in and bring up no air-bag warnings, so that's problem solved all round.
The post-2013 front seats I was selling on eBay had attracted quite a bit of attention and a few offers, but eventually someone paid the asking price of £180. This was mainly to convince me to leave them with someone to meet a private-courier organised by the buyer, which was a bit of an inconvenience, but they sold regardless. This leaves me £40 in profit after all thanks to the error, save the fuel money travelling to Northampton.
