The Stafford Hotel in St James does a lovely roast dinner on Sundays (and it's fixed price on Wednesdays, I believe0. Their American Bar is a cosy nook for dining as well and one can order off the dining room menu if it's not busy. It's lke dining in a lovely country house.
It's pricey, but you can check out the website if you want more information.

Jane in PHL

www.thestaffordhotel.co.uk