I appreciate that you have the MK3 socket with built in filter but did you try removing the bottom half of master and connecting your router with a filter to see if that works - eliminates a problem with the MK3 and also your internal wiring
The MK3 is very easily damaged by storms or electrical surges. You may need to totally remove the whole of the front section to reveal the original BT Test socket behind.
that is up to you but using test socket would confirm if problem is MK3 or hopefully outside your property and less likely to generate a charge
I askedBThelpdesk specifically the question about potential damage as a result of the power cut and he told me that his tests show that this isn't the likely solution
There is no way that they can test that remotely.
The line test only proves the connection as far as the test socket. If that did not show a fault, then that is why they said that.
Can you get dial-tone on the bottom section of the MK3?