Some good news.
The line was replaced this morning and some of the connectors were also discovered to be faulty (where the dropwire reaches the house).
They ran their usual suite of checks and the line had a max connection speed of 27mbit on his hand held device. However, due to a number of CRC and FEC errors he could only get a max speed of around 22mbit (typical of the speed I have seen for the past 3 years) - he was seeing these errors before the line was replaced.
So while the replacement line has solved an issue, they believe that there could be some electrical interference in the area (REIN) causing the errors as a neighbour was also having a similar issue. Mine and my neighbours faults will now be combined and a REIN engineer should be in touch.
Noise margin tends to be ok from 8am to 6pm, then will drop after this. So I guess it's a matter of finding the source of this interference - My initial thought was street lights but there's nothing in close proximity to the lines from what I can see (only 1 on the main road near where the phone line comes out the ground).
Saga sort of continues