... is back up! The offer is 15.15%! http://www.topcashback.co.uk/hilton-hotels-secret-offer/ I am bookmarking all these secret pages for future reference. This is the only one I know of so far.
Just booked a 2 night stay in London with that, also paid with Amex for the spend £250, get £50 back. Add double points and Diamond bonuses, that's quite profitable few days away.
This offer is still running a month on. Current offer 9.09% for standard members. The 'public' rate is 6.06%.