mtacchi - based on all the plaudits the Park Lane is receiving, it sounds like your rate is a steal! Definitely interested in your impressions of the property, the more input the better.

As for the Sheraton Heathrow, I was there recently (due to some misguided notion that it would be easier to make my early morning flight than staying an extra night in the city). A most disappointing property. Then again I've yet to find an airport hotel that is anything more.

If you can avoid it I would. Unless of course you are chasing a bonus promotion of some kind.