[ferretuk]

It looks like Virgin became aware at around 17:30, judging by this post to virginmedia.announce.internet:


Please find details below of this service affecting issue.


Raised : 30 Jan, 2010 17:37
Name : Colin Thornton, Outage Surveillence Team
Ticket Number : 1182464
Estimated Resolution : 4 Hours
Status : Updated
Priority : P1
Services Affected: Broadband Internet

Description :
Some customers nationally may currently be
experiencing an intermittent connection to some
web sites.
Our engineers are now investigating this issue.
Virgin Media apologise for any inconvenience
caused.

Virgin Media Technical Support


Updates:
--------


Date : 30 Jan, 2010 20:24
Name : Colin Thornton
Estimated Resolution : 4 Hours
Status : Updated

Description :
Our engineers have advised that this issue has
been resolved and service restored. Further
checks and monitoring are being conducted to
ascertain the cause of the interruption to
service.

Our apologies for any inconvenience caused.

Virgin Media Technical Support