These things: New Genuine BMW i3 IO1 Set Of Rear All-Weather Floor Mats 51472353819 5056162837157 | eBayThanks for the useful advice. But I think you are talking about the carpet type mats?
I have an issue with the rather strange ‘all weather’ mats that BMW sell as an accessory. They are not rubber but seem to be made of an obscure material with a coating on. Light grey colour - which was a pretty dumb idea in the first place. I have tried a number of cleaners but so far without any success.