l.slater92:No that isn't right.....
OK, sorry I got that wrong.
The first thing you need to do is to understand whether or not the Billing Code applies. Details here. The FAQ is the most useful....http://www.energy-retail.org.uk/customerbilling.html
Next you need to print out the British Gas code of practice on complaints.
The first issue is whether or not British Gas is in breach of the Billing Code by billing you for energy consumed more than 12 months before the date of the "big bill". You need to look at the dates on all the communications and whether any readings on any bill are estimated or actual.
Forgive me but I don't fully understand the "agreement". Was the "agreement" that you would "clear the bill within two years at your own rate" or is it possible that British Gas think the agreement was to pay a fixed monthly payment (and you didn't)? I would say the first interpretation would be quite unusual.
No matter, the first issue is for you do decide whether the Billing Code applies. Next, can you find an issue to raise a formal complaint about? If you can, raise it strictly in accordance with the complaints procedure. If you write make sure you use "signed for" delivery. If you phone make very careful notes about who you spoke with, the exact time and date and what you discussed and/or agreed. You can complain about anything, but it's better to have something you can make stick. For example two phone calls or letters saying different things, broken promises, non compliance with the Billing Code, etc
Meanwhile get a handle on your consumption by taking daily meter readings for a week, then weekly readings for a month. Should you ever receive a bill, check that any estimated reading on the bill is close to the actual reading on the meter.
To return to the question you posed in your OP, not unusual for the industry, not unusual for British Gas.