Thanks for your advice.
As I am being changed to new supplier on the 20th July, I am wary of changing/risk in case there is no immediate improvement.
Have two days to go.
Speed continues to drop from initial 50 Mbps.
Today 16th July it is now 34 Mbps.

It's a pity you seem unwilling to name your new ISP. We may be able to advise on its fault-fixing procedures compared to BT Consumer's.

Am I right in assuming you still haven't tried turning off your modem/router for a bit then restarting it? That is the standard action every person advising you on these forums would have taken ages ago.

If that is all that is needed, we will never know, as your change to a new modem/router with your new ISP will de facto achieve the result that would achieve if that is where the cause of the slowdown lies.

If however there is a fault on the line, your new ISP may be either good or poor at fixing it.

Edited by RobertoS (Wed 18-Jul-18 14:06:36)

I still don't see what your new contract promises in the way of speed

Can you tell us which ISP you have joined and what speeds they promise?

Many ISP contracts offer speeds around 35Mbps

It is becoming clear the OP came to the forums for advice and is adamantly rejecting any that is given.

[shakes head in despair]