and why would your service provider be giving you a wireless password?

Unless you are totally wireless and have no wired connection to your ISP but that does not seem to be the case with this company

Your wireless password should be determined by you and not your ISP and should be part of the setup/configuration of the wireless router.
Connect your Mac directly to the modem/router with a length of ethernet cable, launch Safari and browse to and change your security settings.

Was this modem/router supplied by TalkTalk or was it one you purchased. Note that some routers do not work with that system.
These do:
DG834G - Version 3, Huawei HG520, Philips SNA5630NS, Belkin G Wireless

A few more details would help