The speed quoted by your ISP is a maximum, and to achieve it you need to ensure that your modem and telephone cabling are in good condition and that filters are fitted to all used telephone points. You will also only get full speed when no-one is sharing your ADSL bandwidth. Your account may well have a contention ratio of 50:1, which means that at peak times it is possible for 50 users to be sharing the bandwidth, although in reality the actual figure is rarely anything like that high. Speed figures supplied by your computer are generally fairly meaningless. I find these online tests fairly reliable click here and click here
When Tiscali first upgraded my connection I didn't get any measured improvement so I emailed their customer support desk. They replied that they would "make some adjustments" at their end. Since then I have almost always had excellent speeds.