Are any other talktalk users able to connect with incorrect username /passwords ?
Yes, it's been like that for some time. PPPoA credentials are ignored, they can control user access by controlling the port at the exchange.
This implies that you should be able to use potentially better, but locked down routers from other ISPs
Line: Length 250 metres
Modem router: Buffalo wbmr-hp-g300h + LinkSys wg54g both running dd-wrt firmware
= Down 19418 kbps Up 1019 kbps
WBC 20Mbps Down: 22010 kbps Up: 1019 kbps
ADSLMax 8Mbps Down: 8128 kbps Up: 448 kbps