Not sure if this would apply to any other stores. and may only be relevant to local peeps.
(FYI, this store is just off the A48, @ junction 30 of the M4.)
Have checked and this DOES work.
Spend £1 @ Nisa supermarket at Pentwyn Store Cardiff and recieve 10p off per litre of petrol. Current price is £1.22 per litre so effective at £1.12 per litre. Receipt is stamped but petrol must be bought same day.
Please add other stores with this offer if applicable.