When you say you've tried the Master Socket tutorial on DSL Zone, do you mean you've just tried the test socket or you've removed the un-necessary wiring as well? The problem you describe is either faulty wiring or the existing BT faceplate is faulty.
Do you usually connect through the Master socket or an extension off it?
Do you have other phone extensions, if not, why not just connect through the test socket. I did this for a couple of weeks after being regraded as I was having problems with the extension I use for the router.
Buying the filtered faceplate will most likely help. If you do decide to go for the filtered faceplate, consider this one click here which comes highly recommended. I have the dangly type filter by the same company.