Slow speed 4 weeks. TalkTalk & BT both say can't fix. Ideas?

Hi, I've been having problems with very slow broadband speeds for 4 weeks now. Have faster fibre from TalkTalk and been getting reliable 50Mb for 3 years. Dropped to 5 or 6Mb 4 weeks ago. So far 4 engineer visits. New router, new master socket, new cables. Router connected direct to master socket and using ethernet to laptop for tests. Also tested and same speed on phone using wifi. No extension lines on phone line.
Line into house replaced and switched line from pole to green box. Changed ports and then done a lift and switch. BT saying nothing more they can change and that their tests show 60Mb speed at master socket.
TalkTalk saying nothing more they can do. Their last suggestion was unplug my phone and see if works with no phone connected for 48hrs, made no difference.
Any suggestions as to how I can get a resolution?
Thanks

What speed is the TalkTalk router saying you are connected at?

It should be pretty much the same as the 60 Mbps engineers say is present at the master socket.

If it is around 60 Mbps, then it may be congestion on the TalkTalk network that is the issue and therefore you need to see if the speeds are bad across the full 24 hour period

Just incidentally,is this Paul Smith you?

Back on topic, have you changed the way you measure speed, and is the speed loss affecting browsing and forum use?

You say BT have said you have 60Mb at the master socket. Have you tried plugging just your router in at the master socket and checking to see connection stats? If they can get 60Mb at the master then you should be able to as well and if that is the case then it would be something else in your house causing the problem.