I live fairly close by and find Shell the cheapest in Kilmainham, however I notice that the lads on the far side of the pond, particularly Scotland seem to be looking for a high octane Shell petrol, which I haven't seen here (Ireland)
Many years ago in a former life I used to work on Rovers (P5 and P6). The Rover 3.5 coupe (late sixties mid seventies) required 5 star, which was 99 Octane, which became unavailable. 98 Octane caused pinking or pre-ignition. This is when the fuel air mix in the cylinder head after being compressed by the piston explodes instead of combusting in a controlled fashion, it was also called knocking because if the big end bearings were worn the shock of the combustion occuring too many degrees before TDC caused the bearing to come into contact with the crankshaft. The solution in the Rover V8 was to retard the ignition timing by 2-3 degrees. This led to a slight loss of power but did work. Pinking or pre-ignition usually occurs at low engine speeds 1000 to 1800 on full throttle. People with Rover coupe's added octane boosters to cure the problem and having driven some of them with the octane booster I can confirm that it did work. But it was expensive!!
PS 96D47474, what is yours