I've now used a tank of Shell VPower super diesel & plan to stick with it.
It costs 5% more but gives me 5% more economy, lots more power & a much smoother drive.
I only have to touch the accelerator & she just takes off!
I imagine my emissions are much lower (no MOT worries) & it will only continue to get cleaner the more I use it.
What's not to like?
Many thanks to those who helped me in this thread. The least I can do is let you know the end result: an engine that feels like it has done 10k miles rather than the actual 141k.