Morrisons fuel
 

Anyone in the UK and run a Diesel give Morrisons supermarket fuel a go,
downside is it froths a bit so is hard to brim, upside is it gives a very
noticeable increase in mpg on my Clio.

I run a petrol car so cannot help, but twice in the last couple of months I am convinced that my local Morrisons gave short measure when I filled my tank. So much so that I am now driving to the other side of town to fill up at Asda.
Regarding your improved mpg, I think that is more likely down to the incredible hot weather we have been experiencing. My mpg is the best it has ever been, during this hot dry spell.

I have monitored the mpg both winter and summer and though there is a noticeable drop in cold weather the Morrisons is still better than other brands.
I only run petrol in bikes but one a scooter is better on BP Ultra or what ever they call it but it's the only one other bikes I have noticed no difference.

I too noticed an improvement in mpg running Shell V-Power as against supermarket 95 RON, but that is because it has a higher octane (at 97 RON). Unfortunately, the mpg improvement didn't offset the increased hike in price, so I reverted back to supermarket fuels.

When you adjust your ScanGauge after a top up, do you set the Gallons/Litres to what the pump receipt says it took? I was advised to adjust the ScanGauge to half the difference, so if the ScanGauge says 6 gallons and the receipt says it took 6.4 gallons, I re-calibrate the ScanGauge at 6.2 gallons. Seemingly, this makes for long term better and more accurate ScanGauge readings. Nothing to do with your original query, just an aside for ScanGauge users.

Yes that's what I do set it to half otherwise you end up chasing the mpg up and down,
it is usually about 1.5 mpg up or down from correct as good as the factory fitted ones
on my bikes.