Thank you for your reply.
BT Openreach has been and tested the line. His tester showed DOWNlink of 3.2Mbps and syncing at 2.5Mbps.
I have checked the router stats and both the Uplink and DOWNlink line attenuation boxes have restored to normal loss before the low DOWNlink speed occurred last week.
It appears that breaking down the line to test it has knocked off the problem.
I will see how it goes for the rest of this week.
Thank you for your assistance.