No difference at all. Morrisons and other supermarkets do not refine oil.
They can offset the costs because they don't rely on fuel for their sole profits.
http://www.whatcar.co...iesel-inferior/217130
These days it's all the same stuff fro the nearest refinery. In the past though there have been cases of supermarkets getting some cheaper petrol from whatever source and causing engine damage, Google the "Nikasil bore issue" for BMW's. I never buy supermarket petrol simply because you never know when they are going to get another tanker full of some sort of dodgy petrol. Having said that it's pretty unlikely.