The only thing I think about and easy to do (if possible from TalkTalk side), call them maybe little explain of the situation and then ask the customer service rep. to do a resync. for the service at his/her end while that is being done you should see the DSL light on modem/router went out then starting flashing again till the line fully resync. if that happens then for sure 100% the port is correct both on TalkTalk and OR but for some reason PPPoE connection is not working, if the modem/router did not show a resync. is happening then the line port is mixed and not the same on both sides.
One last thing, I had an issue with my ISP (non UK) which is the same as yours exactly but the reason was a new PPPoE username/password due to a change happened in system upon my request but no one told me and yes line sync. normally but no IP (no internet at all) until I called them one time and by coincidence a customer service rep. who is knowing what happened told me about it and bingo everything worked fine ever since, check this matter with them if something changed in the configuration it might be your case.