Looking for a LNF rate on the JW Grosvenor House in London for 1 night 9 Dec

Marriott .com showing as 305.00 GBP including taxes for a non-refundable rate

I've actually found a 3rd party site which prices to 291.82 GBP but, amazingly, I'd only lose £30 for a cancellation before 5 Dec so although the conditions are better than those at Marriott I'm guessing they won't honor the LNF promise.

All help gratefully received. Thanks.