P0251 is a bad code to get unfortunately. It could be the fuel pump, the IMV, the injectors, or a host of other things. It's always fuelling related

I'd put money on it being nothing to do with supermarket fuel - and no amount of additives/posh fuels will cure a P0251 code. It's definitely worth changing the fuel filter and inspecting the contents of the filter chamber for metal fillings (as said above) If there are metal particles in there, it's the pump, and it's time to scrap the car, as you'll need to replace the pump, and clean out the lines/tank and probably replace the injectors.

My advice (based on extensive work on my Mondeo) would be to take it to a Delphi specialist (NOT a main dealer) and let them diagnose it. It's very very easy to chase your tail and spend a shit load on parts which make no difference at all. Get it diagnosed properly is by far the best advice I can give.