If you've not already done so (and assuming that you've got the correct type of phone socket) remove the bell wire. (It causes interference to your broadband signal):
http://docs.google.co...DExMjllMGMxOTVmNTc5Mg
If your router isn't plugged into the master socket (i.e. if it's on an extension), move it so that it's plugged straight into the master socket.
If you're using a wireless connection between your router and your computer, where a wired connection is possible, change to the wired connection.
If you're stuck with a wireless connection, keep the router and the computer away from all other electrical equipment (as far as is possible) and minimise the distance between the two.
Run a ping test:
http://pingtest.net/
If you don't get an 'A' or 'B' result, get your phone provider to check the line.
But, at the end of the day (Aargh!!! Have I really just used that horrid American phrase?), you probably need to switch to an ISP with a decent 'contention ratio'. (That's the number of people who share a common connection to the internet). Most 'budget' ISPs have contention ratios of around 50 to 1. You need to be looking for a contention ratio of 20 to 1 or better. The ISPs which offer decent contention ratios cost more but you'll get a more reliable connection to the internet.
Chris