<a class="postlink" href="http://www.tunehotels.com/" onclick="window.open(this.href);return false;">http://www.tunehotels.com/</a>
this lot have 3 in London.
The Westminster one is literally next door to Lambeth tube station.
Rates vary but about £85 a night.
Very new, open a year. Rooms are very small but en-suite and you pay extra for TV etc.. but as a base they are fine.
I stayed there at Xmas with my family.
The Travelodge at Covent Garden is also ideal for central sights.