I finally managed to get a tank full of BP Ultimate Diesel into the car (20L left, so 130L of fresh fuel) and within a couple of minutes' driving I'd noticed that she was smoother, quieter and seemed a little more responsive. It wasn't until I reached a hill where my car normally kicks back a gear under cruise control - and didn't this time - that I was convinced that what I thought was slightly more power actually was more power.
BP Ultimate is supposed to have a cetane rating of 55 (a bit higher than the competitors) and have added detergents and antifoaming agents. The higher cetane rating means that the fuel will start a more thorough ignition process a little earlier. I don't know how this will affect chips like Unichip that alter injection timing by playing with the CAS signal, but perhaps someone who HAS a Unichip (or similar, it MUST change injector timing to be of concern) might let us know if there's a difference - for example, a rise in diesel knock because the fuel is burning sooner than expected.
Still, about 65km done on this tank and I'm sure that my car is quieter. Tomorrow morning I'll start her again and will see if it is quieter when cold.