Ok, two experiences here.
The first was ours, where we ended up not getting any gas bills at all for the first 18 months after we moved in (because the gas company screwed up and my wife and I both thought the other one was paying for the gas). The gas company were happy to accept that they were at fault and spread the outstanding amount over a couple of years (I think they just put us on their budget scheme at 20% or so above the normal payment level).
The second was my ex-bosses. He moved into a new house and despite his wife being at home most of the time the gas company failed to read their meter for over two years, giving them estimated bills the whole time. When they finally read it and gave them a non estimated bill there was a £1,000 gap.
He spent several months arguing with the gas company, asking them to prove to him that they had used that much gas. In the end the company folded, the lack of meter readings meant that they always backed down when he asked for proof. He didn't pay any of the £1,000.
I'm pretty sure that the gas company has a legal responsibility to read your meter after a certain period, I'm sure that is what I have seen on the cards they stick through the door after failing to read a couple of times.
In our current house, the only way we manage to get a non-estimated reading of either gas or electrity is to read it ourselves. I doesn't matter how many times they come to read the meters, it never makes it onto our bills.
This doesn't suprise me though, as I know someone who used to develop software for the UK domestic gas supply market. According to them the whole meter number scheme is a mess, the main problems being that the supposedly unique meter number isn't unique and that developers of new houses rarely manage to supply the correct meter number with the correct address when they connect to the gas supply.
Edit:
According to this site:
http://www.adviceguide.org.uk/n6w/index/.../gas_supply.htm
they should be inspecting and testing your meter every 2 years
P.S. gas prices (and calorific values) of course change over time, so they have no way of knowing when the missing gas was used. I wonder which part of the period they assume it was used in ?