You need to be sure that both upline and downline speeds are within tolerances - typically 150 kbps upline and 400 kbps downline are the minimums above which you may have difficulty getting the ISP to accept that there is a problem.  You will need to log at least three speed test results, taken at different times of the day and ideally different days of the week.  These may be run from sites such as:

http://www.wugnet.com/myspeed/speedtest.asp  - requires Java to be installed

http://www.thinkbroadband.com/speedtest.html  - requires Java to be installed

http://www.speakeasy.net/speedtest  - requires Adobe Flash (not Shockwave) to be installed

Your own ISP's website, e.g. PlusNet old or PlusNet new or Waitrose

If your ISP's line is delivered by BT Wholesale (as opposed to an LLU provider) you can use their speed tester by visiting http://speedtest.btwholesale.com/ (requires Java to be installed); a good explanation for beginners is available here.  To exonerate your ISP from being culpable, disconnect your router/modem from their system and connect to speedtester@speedtest_domain with a password of welcome before running this test.

You can find out roughly the speed you can expect at your master socket by checking with BT or  Sam or  Kitz

Make sure your router is connected to the master socket rather than an extension.  If the computer is not co-located then use Ethernet cable to take the signal to the computer.  Phone extension cables and wireless will cause significant loss of speed.

What affects speed ?

Distance from the exchange

The most crucial factor to affect what speed you’re capable of getting is the distance of the cable between your router (including any extension cables) and the BT exchange - the shorter the distance, the faster your broadband can be.   This applies only to broadband via a standard phone line (ADSL broadband) and not to cable broadband.

BT estimates only 78% of its lines can support broadband rates of up to 4Mbps or higher, and only people living close to an exchange may actually be able to get speeds of up to 8Mbps.

Contention ratio

Speed will also be affected by the number of people sharing the same connection to the exchange as you, and also the number of people sharing the same connection at the ISP's facility.  It’s often set at 50:1 but some providers (and ‘business packages’) offer lower ratios, giving faster access.

Time of day

This is linked to the contention ratio. More people tend to be online between 6pm and 11pm (peak times for many ISPs), which means speeds may be slower at these times.

The quality of your modem router and cables

Any wire between your master socket and your router must be high quality.  Telephone extension cables contain a high aluminium content and this allows cross talk which degrades a broadband signal.

The number of computers in your house that are sharing the internet connection

The capacity of the website and webpage you’re looking at

If lots of people try to access the same website or webpage at the same time it will take longer to download the page or a page from the site.