Hi oompah, Tiscali will be unable to improve your speed. You will get the same speed for any ADSL Max product (up to 8, 16mb etc). The max your line can handle is currently the speed that your router is connected to the ISP (you can check by logging on to your router).
The distance from the exchange is almost impossible for the end user to calculate, as although you may live next door to the exchange, the connection may find its way around the village and come back into you last.
My advice to get the max possible speed it to do the following:
1. Remove any other cables in your BT socket other than terminals 2 and 5. More are connected, but providing you have filters on all connected devices in your home, you on;y need 2 and 5.
2. Ensure you have a filter installed on all telephones, routers, fax machines, sky etc
3. Run TCPOptimizer from speedguide.net to ensure that your PC is not preventing you from reaching the max download speed.
4. If possible, connect your router to the master socket (the first socket in your house where BT comes in, all other extensions are cabled from this).
If you do that, you may find that you improve your speed. Mine went from 3.5MB to just under 6MB (i live quite far away from the exchange). And that is on an extension.
Finally, the speed of your connection is best derived from your router. Dont use download speed testers as the speed you get will fluctuate slightly depending on the time of day (as gas4you says), which site you visit etc. But your router will connect to your ISP at the highest speed possible.