Just out of interest, why not Fuel the Aircraft up to the nearest litre, that way you get what your exactly paying for, don"t give them a chance to round it up.
Secondly, come down to Newquay Airport, Cornwall instead, we have a New Hard Standing GA Park not far from the Tower, a friendly handling agent, Friendly Refuelling Team, to cater for your Refuelling needs from around 06:00 until very late and best of all, so long as you stay outside the Restricted Zone, (which you do) the Security is less Intrusive.