In 2012, Hertz at MAN rented me a diesel SUV (all they could give me as a poor proxy for my booked automatic sedan) that:

• had no "diesel only" sticker on or around the fuel tank cap;
• had no "diesel only" sticker on the dashboard; and
• had no manual (or any other guidance) in the glove compartment.

Additionally, the sole checkout agent working a long line of customers gave no indication that the vehicle was a diesel.

Happily, the UK gasoline pump nozzles won't fit into a diesel, or I would have made the same mistake as you.

If any of the same experiences applied in your case, I would argue strenuously that the mistake was caused in part by Hertz's negligence. As it is, because of refueling uncertainties I returned the vehicle without refilling, and had refilling charges defrayed.