I have to agree with Rupert – in the evenings I am not using the bikes for commuting but for short trips around central London (shopping and visiting venues) and it is true the bikes do seem to all but disappear, so that between 10 and 11.30pm when they are needed there is very little to be found.
More bikes are needed around the major educational buildings (e.g. Kings, ULU, the various halls of residence in W1), because this brings bikes into the central areas in balance to commuters taking them out. I have seen a good flow of bikes throughout the day outside Birkbeck who also cater to the evening crowd. (and central halls of residence) but they need to put some thought into where they are placing them, and of course adding much needed capacity in these areas.