Question Author
Thanks all for your helpful replies.
Just to confirm I am using the internet through cable and not through the phone line.
When I tried to use the internet this morning, it was dead. The lights on the modem were flashing (as they often do) and the connection was only restored when I switched it off for about 10 minutes. The 30 second power off/power on that Virgin recommends did nothing. And when it was up and running again, it was still unbearably slow.
I telephoned Virgin Media three times this morning and was getting extremely frustrated with their support line. All calls were routed to India and none of the three people offered a great deal of support. The first man told me to go to a speed testing website and took the download (about 250kb) and upload (about 400kb) speeds. He then went on to say that there was a lot of traffic and they were trying to offer a fair speed to all of their customers. He estimated that within a week, my connection should be back to normal.
I must admit, I am sceptical. As mentioned previously, this problem has been going on now for about 3 weeks and it feels like I am using dialup. Surely, Virgin Media would not slow everybody's internet down for a whole month whilst they try to work out a solution.
For now, I'm going to give up with this. I'll wait a week and if there is no change, I'll be switching to a more reliable ISP.
Thanks again all.