I fixed the problem myself. I found that the external BT cable is actually joined together using a connector strip before it comes into my house. Looks like over time the weather has gotten to it and one of the connector screws had rusted away. I replaced the connector strip and it's all good now.
However, I fixed this last night around 6pm, so I'm not sure why you saw a fault when you ran a line test this morning?
Thanks for your reply.
Would you still like me to raise this over to Openreach to investigate the possible line issue or would you prefer to monitor the connection first?
I had already raised the problem through the TalkTalk Twitter helpline and an engineer had been requested. After it was fixed I told them to cancel the Openreach engineer. All good now, thanks anyway.