A and C
I looked into this about a year ago and there is a catch 22. The price comes down significantly if you buy 5000+liters. Below that you save maybe 5-10p/liter For 5000 liters the saving is ~15p/litre.
There are regulations about how much you can tow on the road. I think the limit is about 600 litres. To tow more than that you need to go on a special course and get a certificate.
So the only way to save money is to have a large static tanker and get the fuel company to come to you. This works as long as you use enough to keep it fresh and you have security etc.
Let me know if you find out anything different.