Well I have ditched using V-Power. There was no improvement in smoothness this time round, and any improvement in mpg could be attributed to "noise".
The previous time I tried it, the car had been running on supermarket fuel for who knows how long, so the additives may have given it a "blaw through". However, in the interim, I have stuck two bottles of Redex through the system so maybe my fuel system is as clean as it is going to get.
Today I put Shell Fuelsave in, but I think I will go back to fuelling at Morrisons (at least until Asda reopens their filling station). Even when Morrisons sold me short measure (and I am still convinced they did - on two occasions), it was still better value than buying my fuel from the Shell station. Only problem is, it messes up my mpg numbers.