Unfortunately, the Which? report only identified and focussed on one area of the benefit of high(er) octane fuels. With forced induction pinking or knock is much more likely so using too low an octane can (I'd say will, but I'm trying to be unbiased) cause damage on a molecular level to the internals of the combustion chamber and adjacent areas (water/oil channels, camshaft bearings, valve guides etc al) because of the early detonation "shock wave" effect.
Modern engines have a knock sensor (or many) listing for this and will when it can, advance the injection timing to compensate, but this will effect your efficiency of the 4 stroke cycle as there is always an "optimum".
The farther up the performance curve you go, the more important this becomes. I would advise anyone with a JCW, and probably MCS owners to choose a higher octane fuel if they can, but if you are as finicky as me engineering wise, you probably do anyway. If you modify your car other than simple exhaust/induction, most tuners would insist on it to protect their hard work and maximise your gains.
As for diesel, well. This is my area of real (read: professional) expertise although somewhat outdated (I stopped being a professional oily person before 2000), but nevertheless, I have a degree in Engineering (Marine) and spent many years hitting 30,000hp engines with hammers.
So, to my point. A diesel engine will burn pretty much anything once it is warm as long as it's slightly volatile so the "quality" of DERV is somewhat of a misnomer. However, the crunch comes with modern diesel cars as they have a number of factors that require a "nice" fuel.
Audi and BMW are particularly good as getting magnificent performance and efficiency from their TDi's, and this is primarily down to the dynamics of their injectors and the atomisation of the fuel they produce in the combustion chamber, and this is 90% down to the injector nozzle itself.
When your engine is COLD, DERV is an unfriendly liquid and takes real encouragement to burn, hence glow plugs etc. If your injector nozzles are dirty and gummed up, this will be harder and you'll get big, fat droplets (relatively) rather than the superbly engineered mist engineers like me love. This can cause a "lean" burn at start up and this can put real strain on your valve seats over (admittedly pretty long periods of) time. So, DERV with cleaners in help prevent this. It would be better if they said "V-Power Diesel maintains your engine's efficiency for longer" as this would probably be a more accurate statement!
There you go, that's my lesson, shoot me down at your will!!