Betfair Customer Services have made the following announcement:-
Please be aware that, due to urgent maintenance on our databases, between 06:30 GMT & 07:00 GMT on Tuesday 20th November 2012 you may experience an interruption in the capability to place/cancel bets on the UK exchange (and associated channels).
For this reason, please ensure that you have managed your bets in advance of this time period where possible.
We apologise for the inconvenience, and aim to complete this work as quickly as possible within this time frame.