The technicians from Open Reach called today and were able to fix the slowing down of download data rate. They discovered several possible faults [any of which could have contributed to the slow down] to include corrosion in an outside junction box, a technical issue regarding the number of connecting wires, and an ageing master socket [it was over 35 years old]. They also had a tinker with the green cabinet.
The max potential download speed is now 40 Mb/s and the actual rate is 35.9 Mb/s. This is a significant improvement and puts me well within Talktalk`s estimate for my line.
Thank you for your assistance in resolving this problem and for the reference to Open Reach. For me it shows the value of using the forum to solve problems.