but nearest shell garage is 15 miles from me
on the way back hopefully i'll find some

Tesco's 99RON Momentum is made by Greenergy.
It's not a bad fuel ie. it meets statutory minimum standards.
But, the octane rating is achieved by dosing it with up to 5% by volume bioethanol. Other than this amount of spirit alcohol it is the same stuff as Tesco's basic 95 RON unleaded. Alcohol is very astringent and drying and if you want to run your car on the stuff then best to buy one that's designed to run on it. Hence Saab's Biopower models.
Shell V-Power is a much more 'complete' fuel with higher quality base stocks. Less need for Octane improvers yet it also has a very efficient detergent package.