Wednesday 20 January 2010 7.58pm
Thanks for the feedback. I managed to hold of phones from Orange, Vodafone and O2. No signal on O2, questionable signal on Orange, good signal on Vodafone. Three (my current provider) have a faultless signal, but they don't offer the newer iPhones. All of the operators show 'Excellent' coverage on their websites :-/
Incidentally, I found the following website that shows locations of the mobile masts and their operators. Vodafone masts are all over the place, and Three have one on top of the Travelodge on Tower Bridge Road
which is pretty close to my flat. Orange and O2 are a little further away. Hope this site is useful to others.