I drove the 8 miles from my office to the BP on watford way with a leaden right foot, to try and burn up the fuel I had left in the tank - the fuel light came on just as I pulled onto the forecourt

Filled up with Ultimate (which was 3p/litre more expensive, 82.9 vs 79.9 for reg diesel).
Now.....I tried to be subjective about the improvements, but knowing that I should be expecting an improvement, I couldn't figure out if it was a real difference or not.
It felt much smoother, quiter at cruise and boost ramped up more easily/quicker. It even appears to be more economical. However, this wasn't a scientific test, so for the benefit of my imagination, and the forum, I will re-fill and conduct some in-gear tests.
Then, later in the week when I need fuel again, I'll re-fill with regular and we can compare the figures - this is the only way I'll KNOW there is a difference.