At what time of the day did you do your speed checks on ADSL Guide?
You'll generally find that the fastest speeds are in the morning (and by morning I mean around 7am not 11:58am) and the slowest in the evening (after 6pm when people are home from work and beavering away on the web).
Obviously, also, your local contention ratios and distance from the BT exchange will have an effect.
I recently upgraded with Tiscali from 512mb to 1Mb to 2Mb.
My 2Mb connection now averages (across the day)downstream speeds of 1,899 Kbps.
I suggest you compare your speeds across the day before approaching Tiscali. Also, if you are using the Sagem Fast modem (which I am), there will be no need to update the drivers.