Sorry I can't help with your problem, but just to point out that if an engineer was currently fiddling with your connection then I wouldn't be surprised if tech support saw incorrect signal levels.
When your connection is working you might try posting on the virginmedia.support.broadband.cable newsgroup (
http://www.virginmedia.com/help/cable/newsgroups/) to have your issue dealt with by British 2nd line technicians.
Failing that you should then follow VM's complaints procedure (outlined in the Code of Practice available here:
http://allyours.virginmedia.com/html/legal/index.html).