Just got of the phone with technical support. Apparently some catastrophe has taken out the 3G data network. They have no idea when it will be back up and running (could be days). If you switch off 3G you may be able to access the internet albeit a bit slower than usual.
Also on the O₂ forum: http://customerforum.o2.co.uk/viewtopic.php?t=8502
Also on the O₂ forum: http://customerforum.o2.co.uk/viewtopic.php?t=8502