Problem has been fixed and addressing works. In all it looks like their router was out for about 24 hours.
(I wonder if it simply needed rebooting!)
---------- Post added at 16:28 ---------- Previous post was at 16:12 ----------
Thanks Kwikbreaks for info about Virginboards. Very interesting.
Although the problem was reported there as well as my report here, no VM staff replied to either post, so I'm not sure information posted on either board would be read by VM staff quickly.
Incidentally, reporting the technical fault to VirginMedia by telephone was problematic (to say the least) - I just kept getting transferred to a call centre in India who had little expertise in the internet (or English comprehension for that matter). If anyone out there knows of an efficient way to report VirginMedia network problems in future, I'd be grateful to hear it.