Unless you have Monday off and are going with the sole intention of necking as much champagne as possible with some friends, then I really don't find The Westin brunch particularly great. There's actually too much of it and none of the food is particularly outstanding.
We are huge fans of FEAST and their semi-buffet which runs lunch and brunch times. There is an a la carte main course supported by a starter and desert buffet and it works brilliantly. Unlike most places they just focus on doing a few things well, rather than a lot of mediocre offerings. Champagne is 241 at weekends too if you fancy a drink. 'Trendy' more than upscale however.
Cafe Noir at The Traders is also a dark horse and very underrated. Really don't visit as often as we should.
Not a buffet but the Capital M brunch is excellent value and a fantastic location for a Sunday morning.
Great thread to be resurrected when I'm trying to lose weight