Hugo Brambles:
I've remembered! It was the Great Eastern! I enjoyed it but then again I am a country bumpkin 
To confuse you further, it is now called the Andaz Hotel 
Sophie's suggestion of booking is a good one. When we did a "staycation" a few years ago we booked into Threadneedles Hotel for a weekend - it is really quiet at the weekend as it is in the heart of the City (so lots of deals to be had!), but easy to get to the South Bank and more central London.