i use Bunkercard (BP) through fuelcard services. never had a problem, always give keen price and network is unbeatable. Only available to high volume users though (i think?)
WE use SMART DIESEL and RED When i joined smart they said everyone pays the same price wether you draw 10ltr or 10,000 ltrs its same price ,thats not strictly true my mate has smart he was on a better price than me now some weeks i get better price than him and he has 7artic ive only got 2vans
MRC:
Smart are craft, they put a price up on their website for all to see, thats basically for the folk who dont look around for other prices. A guy i know is on 4p below that price (possibly making a loss on him - not unheard of to make a loss on a select few)
my smart price is less than the website price too 
if I don’t use the card I usually get a phone call with a further reduction 
berewic:
Any comments on DKV?
DKV does seem to get accepted widely in Europe but not to well covered in the UK, We spoke to our rep and got a key fuels card through DKV to use in the UK, without putting anymore deposit down. Not to sure on their fuel prices though.
What DKV does give us is a 24hr help line, which is great if we have probelms with any of our vehicles, especially when abroad.