I used to operate very successfully with the supplied Plusnet Router (N7201571B001163) at a 6dB margin. Lately the bandwidth dropped and I noticed the margin had crept up to 9dB, and for the last fortnight it has been at 12dB, significantly impairing my use of the line (now below 3mbps download).
I'm happier with potential occasional dropouts than with a much slower connection.
Fixed! Go to the fix.