I stand corrected....
I have a countrywide account - south west area, and because that station was out of order I had to use a BP down the A46. I know my 80 litre tank usually takes 45 litres, and I hadnt done my usual mileage, so was somewhat shocked when it took 67 litres, particularly when it was 6p dearer than countrywide!
I questioned the guy on the till, who advised that they often get that, and its due to temp etc... so I left, and called the station manager. She advised that it was okay when last checked, and would get it rechecked that day. Being an untrusting kinda person, I then spoke to the technical guys at countrywide who explained that yes it is all callibrated, 2% maximum error is allowed - against the garage, eg. in the customers favour. He also explained how its done, and it all sounds pretty accurate.
I also spoke to trading standards, who took my number, and I havent heard from them since....
Anyway, on my way back past later I called back in. They had had it checked, and it was nearly spot on - he even showed me the check certificate and the numbers tested. I was impressed that they actually listened and did what they said to be honest!
This was backed up when the following day I filled up at my usual station... it took 8 litres, and I had done 160 miles.... 80MPG - yes please!!
At least I now know they ARE callibrated and you can get it checked. I am also well impressed that BP listened to a moaning customer and took action so quickly