3 things:
1) Listen to Mixture - he knows what he's talking about
2) ....apart from "the worst ADSL ISP", BT are lightyears ahead of TalkTalk/Tiscali.
3) Your line will only ever support as a maximum of what the modem syncs at. No amount of speed test websites will be of use if the sync rate is low. If this is the case, your best bet to influence this is to unscrew the master socket and connect the router into the maintenance port (behind the faceplate). If this shows a significant increase in sync speed, you have problems in your internal home wiring.
If, however, you have a higher sync speed and a very slow speedtester.bt.com result, check the output for a profile rate - as it's this which next influences the throughput from the ISP's end, and it's this which the BT callcentre were talking about a stuck profile (as the profile automatically sets itself to the fastest your line can support, by gradually increasing throughput on new lines until it falls over, and then backing off a bit....which is why ADSL ISPs tell you not to turn off a new install for the first 10 days at least, and it's also why you may notice periodic outages during that time).
HTH.
Mike.