Thanks for the advice! I had found the two threads you had listed but one was for "successes" which this clearly wasn't and the other was for help finding LNF rates, which I had already found. Thanks for pointing out the tax thing. I had just booked two stays in London, and tax is not included there, but you are right, for York it is included and that did make the difference! Its odd to me that for some UK locations Marriott quotes with tax and for others without.
Mike