GTAT,
Stuff is correct. There is a limited period of time where they can bill you and it must be for a fair amount.
Slightly different case but for nearly 4 years Scottish Power had been undercharging me to the tune of £2500 in total by the time they had worked it all out. They contacted me and explained that this had happened but immediately stated that as the fault was with them, they had no intention of attempting to recover this money. In fact, they credited my account with £200 for the inconvenience.
Hope it works out ok for you.