I'd bs inclined to file this original story under "slightly suspicious" as the Express, like any paper I guess, is not averse to a little sensationalism for the sake of sales. I note that here we are halfway through 2011, yet they're muttering about things from 2008 when more recent data are available.
I can't argue about diesel demand having risen but with the way crude oil is made up, and how the cracking/distillation process works it's not so much less worthwhile to produce petrol and diesel specifically, as less worthwhile to refine oil in the UK at all, as we're a more expensive country in general than others - look at the way all our manufacturing as gone overseas as it's cheaper.
Crude oil is full of different types of molecules and to do anything useful with any of it, the oil has to be broken down into these different types. This is where the distillation process comes in, as you can't just decide to turn this batch of oil into petrol and that into diesel and that into avgas, etc. What you end up with is very heavily dependant on the chemical makeup of the oil going in.
Diesel does indeed come off before petrol, being a heavier fraction, but you're going to get both anyway as the crude oil is split into ever smaller types of molecules.
Incidentally, if diesel prices do spike by 15%, then *everything* you could conceivably buy will get more expensive because everything to some extent relies on diesel LGV's to get to you.
Cheerful thought, no??