Mentions a limit here, at the top, for Leicestershire: http://www.leics.gov.uk/index/business/tradingstandards/ts_consumers/petrol_pump_questions_answered.htm" onclick="window.open(this.href);return false;
So allowed up to 0.5% less fuel, and 1% more fuel (if I’ve transcribed that correctly). I don’t know what kind fine is imposed, if they’re found to be outside those limits.
edit:
found this regard fines: http://www.petrolprices.com/blog/supermarket-fined-after-overcharging-on-fuel-for-9-months.html" onclick="window.open(this.href);return false;
I'm confused at the numbers mentioned. 9.2m litres of fuel x £1.30 per litre (I don't know the price of fuel in 2009) = £11.96m / 4% = £478,400. Yet there's a mention of £150,000 and a fine of only £18k. I suspect it's because of the massive tax on fuel, being a major part of the purchase price, but too late in the day for me to work that one out...
edit 2:
Yep, the retailer is making about £0.021 per litre, which is in the ball park of the £150,000 mentioned, and 4% of that is £6k, and they were fined 3 times that = £18k.
But the 4% was the overall fuel volume? I got it wrong. I'm off to bed.