That sounds promising, not sure I 100% understand how the same cable would link back to the switch but I can look into that.
So, the long awaited phone test has taken place - perfectly clear dialling tone and tested the number to check it was the one Plusnet referenced in their setup email, which it was. So phone line working perfectly with Openreach's setup, does that point the finger of blame back in Plusnet's direction...?
Router plugged into Master socket with ADSL filter (all supplied by Plusnet) and tested with multiple routers/filters - broadband not detected light flashing (orange bar with red light)