Thanks all - Based partly on this feedback I have actually decided to switch! I will instead catch the express link into London and stay at the Paddington Hilton.

As I am only Gold with Marriott but Diamond with Hilton (and the rate was only 85GBP), I figure I am better off.

For some reason I don't mind paying 15GBP for the train to London, but 3GBP for the hotel shuttle really kind of bums me out.