An article relating to an oil user in Yorkshire that illustrates that if you can’t be bothered to check prices, you get what the suppliers feel like charging. At least with NOBC you are guaranteed to get the most competitive price going for over 50,000 litres even if you only need 500 litres

http://www.theguardian.com/money/2015/feb/10/northern-energy-price-overcharge-top-up?