AFAIK if your meter is out of cert life you can argue it's not accurate and don't have to pay your bill, tho I've not actually read that anywhere and it's just word of mouth.
They'll install a check meter for a couple of weeks. If your meter is shown to be over 2½% (I think) fast, they will agree an adjustment to your recent bills. If it's within tolerance, you pay for the check meter.