Do you have anything 'wireless' [router, hub etc] connected that someone could be stealing bandwidth from?

Sorry to ask, but can't remember seeing it in the thread. Have you tried rebooting your router, in fact switch everything of and completely disconnect the router (inc network cable). After a minute reconnect router, then start switching everything else back on. I say this cos its probably the first thing Tiscali will say [thats what virgin say].

the ping, or latency is fine at 20ms, i'd only expect the speed issues reported at 750ms or above.

if a router reboot hasn't helped, have you cleared your cache, etc.
also try flushing dns, prob wont do much but it may be that there is a problem resolving the dns to your isp server, however you would prob get other issues if this was the case.
try renewing the adapter (ipconfig /release, followed by ipconfig /renew)
However, the thing that confuses the issue is that your upstraem speed is about what it should be, it only seems to be the downstream that is way off. is it on a bt line, and if so is a microfilter in place, may be worth swapping it for a new one and also testing it with no other adapters in the socket, maybe even get a line test done.

All being said though, you'll prob be best of calling your isp and reporting the problem, they have been known to limit speeds for unknown reasons, and should be able to speed things up a bit.  One last thing, doi you know how far you are from the nearest BT exchange?