I guess I'm too late weighing in but I LOVE the Chancery Court--have stayed there many, many times on business and pleasure. The club floor rooms are lovely and the complimentary lounge buffet breakfast is excellent (I would call it upgraded Continental). The building itself is fascinating as it has retained many of the features (marble staircases and brass) of the former life assurance company. Staff is oustanding. If you are only mildly intrepid and haven't much luggage, the tube line which is half a block away runs straight in and out from Heathrow--run time is around 1hr 20 min, possibly more on Sunday. Watch for Sunday delays. Hotel will arrange a private car for you or a black cab, slightly less than the hire car. CC is an easy walk to Covent Garden and theater district. Tube or buses will take you virtually anywhere else from just outside the door.

Thanks for the input, Cathinjoetown.
I'm actually still on the fence, believe it or not, so your comments are helpful. I'm just worried that I'll be giving up the 'charm' factor at Chancery Court, but location for a whirlwind sightseeing day is important.

For a 07:30 flight on a Sunday I woudl book a car from Justairports (or equivalent) Depending on which hotel you finally decide it'll cost 24 or 30. To me that is worth getting there faster than the tube at that time in the am

With a 7:30 am flight, you'll need to leave quite early. I agree with Wombat7....hire a service to get you there. They'll probably recommend you leave the hotel by 5am....possibly earlier. Seems much easier to do that than to drag around luggage from taxi to train at that time of the morning.

Thanks for the great advice. I tried to submit an online request with JustAirports, but it wouldn't work, so I guess I'll just call them. And I appreciate the heads up for timing. I wasn't quite sure what to expect for a Sunday, but I'm thinking we should probably leave the hotel around 430AM? Definately don't want to miss our flight.

It depends on your comfort zone, whether you are flying business class (with expidited check-in and security), which airline you are flying and how well you know Heathrow.

That early on a Sunday morning I would guess on half an hour to get out to the airport by car. I would be happy leaving at the hotel at 5:30 - which should get you there at 6:00.

Having said that ask Justairports - tell them the time of your flight and they will tell you when they should pick you up.

Let us know which hotel you finally choose. The photos of some of the bedrooms of the Chesterfield on tripadvisor show some quite interesting decor.

It was a very hard decision, but we decided on Chancery Court, based on feedback from a friend who just stayed there, and we'll earn some nice Marriott points for our stay. Since we really only have the one full day, we also felt like we might be in a location more accessible to 'speed sightseeing'.
I'm sure Chesterfield Mayfair is wonderful, and I really hope to have the chance to check it out in the future. Booked JustAirports for our pickup at Heathrow and return. Good advice wombat7 regarding Heathrow. I'm sure Just Airports will know exactly how long to allow. We are using miles for first class, so that might save us time checking in.

I fly out of Terminal 3, travel business class and generally do a web check-in (but have to get my boarding pass at the airport). My average time from check-in desk to lounge is 15 mins - if that is any help.

As I couldn't say enough good things about it, please let me know how you like the Chancery Court. To me, it feels like a 5 star at slightly lower prices. From there it's easy to get down to Picadilly Circus/Leicester Square to grab an on-off double deck tour bus which might be your best chance to see all the main sights in your short time.