I bought my car from a dealer about 3 years ago (a 1.6 petrol c-max, so a different engine), 5 months and 3 weeks later a power steering hose split (common ford fault, depsite it being a generic goodridge hose that probably gets fitted in a lot of cars).
I’d be 100% sure it was fine when they sold it to me. But it was their job to fix it and they did.
I’d not try rejecting just yet, go in in person and press them to fix the problem.
And I’m never convinced by the ‘supermarket fuel did it’ claims. Tesco don’t have any refineries in the UK, and neither do Shell. They might stick an aditive in to do one thing or another but:
a) if they did, don’t you think they’d make more of a song and dance about it, after all it’s costing them money to do.
b) it’s the same basic fuel whichever garage you go to, from whichever refinery was closest/cheepest. If there was some uncombustible crap in it, it would appear at more retailers.