Can joe public buy a barrel of crude oil and make his own petrol?

First you've got to find someone who'll sell you a barrel of crude. A tanker load, yes, but they don't do retail. Then you've got to do your refining. You need to build a fractionating column that won't explode when you heat the oil up. You also need somewhere to build it - councils have inconvenient rules about what you can build in your garden shed. Then when you've finally broken your oil down into its components, you've got to dispose of the stuff you don't want - and no you can't take it to landfill. Having made your own petrol, the government is going to make you pay tax on it. It's probably cheaper down the garage.