If I recall correctly the broadband light should be constantly on, if its flashing it means its trying to establish a connection. If this is happening over and over again then it is likely that it can't detect a broadband signal on the phone line.
Things to try:
1) Try a different Micro Filter
2) If you have a modern master socket where the bottom half of the faceplate unscrews or unclips, remove it. This will disconnect any extension wiring and expose the test socket. Plug the router into the test socket and see if you get a connection. If you get a connection in the test socket, but not with the faceplate in place, then its likely a wiring issue with your extension.
3) If you have a landline phone, check for a dial tone. If you have no dialtone then there may be a break in your phone line somewhere.
4) If you get a dialtone, perform a quiet line test dial 17070 and select option 2. Listen for any pops or crackles on the line which may indicate a fault on the line.
5) Call Plusnet support and advise them of the outcome of tests 3 & 4 and log a fault.
