I don't know guys what kind of fuel you use in the UK and how it is mixed, but this myth of 95/100 is strictly placebo in most of Europe.
In Slovenia, Austria, Germany and Netherlands (did not check other countries, as I have no friends working at gas stations in other countries), the 95 and the premium gasoline (98 or 100) come from the same tank in the ground (which is 95).
They do not have a separate 95, 98/100 and diesel tanks - only 95 and diesel. The additives are added via the handle while pumping gas. This additives are usually of the lowest quality and value. If you want "better cleaning" of your engine, just buy an additive on Amazon once every three months and put in the regular 95 (or get the car to almost empty and fill full tank of 98/100 every 3 months).
This placebo effect is equivalent to you guys sitting in a bar and one of you saying, my iPhone just got really slow. Then all of you would check your phones and believe that your phones also got slow.
How I found out? I was regularly putting 98/100 until I asked my girlfriend to fill the car up once. She saw 95 on the cover and pumped 95. It was the fuck*** same. I only found out because when I was cleaning the car next week, I found the receipt for 95. There is absolutely no more "umph" in the engine if you put in 100 instead of cheaper 95.
Additionally, if you remap you get better fuel economy due to the fact that the car is more responsive so you are easier on the gas pedal on average with the same driving technique as before.