FTTC up to 40 now down to 14

[#1]

After changing over to FTTC, I had stable speeds up to 34Mbps for a month, my woes began when an overheating BT modem was changed and the speeds declined steadily now to 14Mbps. 3 engineer visits in 3 weeks has found no fault on my line and no reason given as to why my speeds have not recovered.
May 10

Today

[#2]

Post the full results you get from the BT Performance Tester.

[#3]

BT test
Download speed achieved during the test was - 12625 Kbps
For your connection, the acceptable range of speeds is 12000-16072 Kbps .
Additional Information:
IP Profile for your line is -16072 Kbps
Upload speed achieved during the test was - 1584 Kbps
Additional Information:
Upstream Rate IP profile on your line is - 2000 Kbps