It's not that cheap for us as the rates go up on 1 April and we have a suite and a connecting room! However, it can be an outstanding bargain as you spotted.

Sat 27 March to Tue 30 March, for example, you can get a Mews Room (same size as the other rooms but inward facing) for £130 + VAT a night, so £153 a night. The Preferred Partner deal means you get a) breakfast for 2 inc room service if required, b) a £57 lunch credit and b) an upgrade at check-in.

This is a steal. Whilst this is not what they tell you on the letter when you check-in, I know from 4-5 visits now that you can take anything on the a la carte menu for breakfast (the cheques usually come to £70-80 for 2 if you have it in-room, and they wipe off the entire amount). You can also split the lunch credit across different days, ie 2 x £15 lunches in the spa cafe x 2 days. Your chance of an upgrade at this quiet time of year is also good. And if the weather is as lovely as it is in London today, its an even better bonus.