First check
http://status-cable.virginmedia.com/vmstatus/summary.do to see if it's a reported issue.
Then I'd post a support enquiry on virginmedia.support.broadband.cable. (details are at
http://www.virginmedia.com/myvirginmedia/newsgroups/ )
Include times of day and duration of outages.
The downstream and upstream statistics from the Cablemodem obtained by visiting 192.168.100.1 (username/password is root/root). Do not include MAC address or serial number.
The Log obtained from the modem on 192.168.100.1.
Response times during office hours are typically around 20 minutes (although can be a bit longer).