Read the article which explains it, but briefly, petrol and diesel (and high octane stuff for jet aviation) are all taken from crude and refined in different ways (i.e. petrol is not diesel which is more refined), by a couple of processes - I won't go into the full details (I am a process engineer working in Oil & Gas
) as it's not needed really - unless people want to know
Basically the refineries we have here don't produce much diesel as it' wasn't so popular back in the 60's and 70's when a lot of the refineries were built, the world wide demand for both diesel and petrol has fallen so refineries are closing all over the world (including here in the UK).
It would cost a fortune to change the refiners to produce diesel so we import more than half we use. The report says (it also say most of what I said above) that if the supply dries up (which it easily could for many reasons) then we would be basically stuffed (OK the BBC doesn't use the term 'stuffed'
Yay build more refineries - more work for me