I have the £50 offer. Confirmed this works as a cumulative spend and on prepaid stays. I had a stay for £150 last week and booked a £109 prepaid stay for March and the statement credit hit my account today.
It worked for me on prepayment no problem. Was worried at first but no issues and got the money back within days.
To save others checking, the offer seems to be for European + UK Hilton hotels only.
I think last time there was someone on here who said it did work for pre payment
Guessing it's not possible to book online and pay at the hotel?