Moral of the story I guess is just don't assume what your being billed is correct and in this case it would never have corrected itself.
Despite sending photographs on 4 occasions and there being visits on 3 occasions by the gas suppliers inspector (or whatever they're called nowadays) the supplier (one of the big 6) "refused" to believe we have an metric meter! Oh no you don't 'cause the register says it's imperial. Oh yes we do 'cause here's a photo and a copy of the report from your inspector. Oh no you don't .........
So, after involving The Ombudsmen the supplier paid compensation (as did The Ombudsmen because they were poor in managing the process) and credited around about £1400 overpayment after arguing for nearly 18 months.
Obviously, we didn't stick around with the supplier but then had issues with the new supplier (one of the smaller guys) who said previous supplier had given them the old imperial meter serial (or whatever it's called) number. E-mailed photo to new supplier same day (Friday) and they requested a visit the following Monday to confirm. All fixed by the Tuesday! They then came and fitted a new meter about a week later as they said the digital display was getting difficult to read.
Smart meter? I think not! I'll tell them the readings every month then I know they've got the correct usage.