that’s under the back billing code and normally only applies when the supplier hasn’t sent a bill to the customer, if @rich-t has been paying/receiving bills in that time it won’t apply
they’ll normally take meter readings either weekly or monthly over the next few months and then bill him on a slightly more accurate estimate for the consumption from last 3 years, that said it’s still worth complaining to them in the hope that they reduce it for the inconvenience