AOL means a LLU switch to TalkTalk
If it was bad before the switch and the same now it does not point to a fault. Presume the phones are working. Who was the engineer that attended working for - Openreach. Question would be what speeds did he see when connected
300/64 for a 27dB attenuation line is what any sensible engineer would consider bad. So suspect you may have just had the line checked for voice.
Solution for me would be this, give ISP an ultimatium, i.e. 7 days further to resolve issue of not getting a connection, and then to be released from contract and migrate using a MAC to an ISP willing to help liase.
Have you tried new microfilters? A bad one may be giving this problem. Also check the ADSL hardware on another ADSL line if you can, routers can fail.