It seems that the faceplate might be the suspicious item. Plusnet ask me to leave the router plugged in via the line filter for 72 hours to confirm. During my other testing the line was fine, but after I plugged the faceplate back in it appears to be dropping.
Would Openreach post me a new face plate to save sending an engineer?
To be honest if it's just the faceplate I don't have an issue leaving it in the master socket as the line isn't used for anything else - and hopefully now Fibre is in the design stage for my cab it might be sorted in the next 12 months and I'll disconnect it anyway.
Zen: Around 6mb down - .8mb up
JohnLewis BB: Around 6mb down - .6mb up
Stechford (CMSTE) Cab 50 - Now starting the Design phase.....