did the guy before you put 20 litres in and the clock wasnt reset?
No, all the Zero's before filling.
Someone did say to me a few years back that when you connect the Nozzle Pump to the Car, if they take a while to reset the Pump and or Activate it ready for fill that during the idle time your tank presure will slowly fill the Pumps Hose up, cant see it myself, even if it did in thoery you would get it back when youve finished filling. I mean end of the day this is Sainsburys not some back yard LPG outfit. 
There's no way you'd get 20 litres into the hose (and your LPG tank has a non return valve in the filler anyway).
If those 20 litres had existed and wouldn't fit in the tank you'd know about it by now. 
Might be worth checking your filler valve is still cutting off at 80% fill. If it is, you've been done, IMHO. 
My thoughts exactly, strangly, on the way back the Contents gauge was reading empty

but this morning went straight back up to full as per norm.
Ive managed to speak to the Manager tonight, he did say he put the Pump OOS pending an Engineers visit, He was quiet under standing with what I informed Him and agreed something wasnt right. I explained that I keep a good record of my Fill Ups, my MPG and what to expect in miles from each Tank fill up, which I do and I can prove, He's response was understanding and asked me to pop in again, I replied I would run the tank empty and see what the Miles and MPG figures are ( you never know) but either way we both agreed I was probably due some money back.
Quite a pleasant Phone Call if Im honest, could have easerly gone the other way.