Just a bit more information on the above....
I've been looking at my network connections, and I've found the following (my internet connection is working, although with the problems described above):
The connection called Tiscali Broadband (ticked as default) is labeled: Disconnected.
A new connection called Wireless network connection 2 is labeled: Connected.
Could this be an explanation for the above behaviour? What should I do about it?
Many thanks for any advice.