Thanks JJW009.

My router (a vigor 2820) is connected to an ADSL Nation XTE-2005 faceplate that's plugged directly into the BT master socket (the BT test socket behind the NTE-5 faceplate). I have previously tested with two other filters (the one I'd been using for a while and a brand new one that came with the router).

There is no internal wiring. I had to have the line installed by Openreach when I moved in as the previous occupants had been on Virgin Media cable. When the line was installed I confirmed that there was no other internal wiring in the house. The only other phone socket anywhere is the old ntl/virgin media one and that's comprehensively disconnected from the cable line that comes into the house anyway.

The NTE-5 master socket is also brand new as it was put in by the Openreach engineer who installed the line in the first place.
I am beginning to wonder if the problem is somewhere in the external BT wiring from the telegraph pole. Unfortunately the landlord or a previous owner, in a fit of madness, put stone cladding on the front of the house and the phone line runs behind this between the point the line meets the eves of the house (on the coiled wire support, whatever that's called) and the socket [i.e. stone cladding/phone wire/brick wall sandwich].
When the engineer came to install/repair the line he didn't have a drill bit long enough to go through the wall + cladding so had to leave that section of line as it was.

That may well be the case. If so, it's still the responsibility of Openreach up to the socket. They charge the same installation fee even if there's no existing wiring, so re-using old cabling to save effort is only acceptable if it works properly.

Given everything you say, there's nothing more you can do except talk to Zen again and clearly explain that you've tested at the test socket and changed all your equipment. Who do you pay the line rental to; Zen or BT? If it's Zen, they should have access to similar diagnostic tools I have for our customers and should be able to arrange for an OR SFI engineer to investigate fully.

Provided the fault is repeatable, you should have no trouble convincing the SFI that it's not your equipment. They do charge a hefty fee if it is, and you have to agree to this prior to a visit. This is why it's important to be so thorough to check everything you possibly can first, and I think you have.

The phone line is supplied by BT.
I'm going to run another couple of tests with a different router to see if it's a peculiarity of the Vigor (though I doubt it somehow).
I have a spare Thomson 585 v7 router sitting about that just happens to be Zens recommended router for home users so I'll see if I can get that to have a heart attack when the phone rings as well.
When I report it to Zen I will be fully armed with a battery of already conducted tests, with screen shots of the router before testing and as it falls over.

Well, I've got a BT Openreach engineer coming this morning allegedly.
As reported in the FPOTD thread however the on-line fault tracker says the engineer has tried to call but couldn't gain access tot he property.
I find this odd. I've been up since about 6:15 when my alarm went off. Had a shower at about 7 and have been pottering about downstairs since. Given the house is open plan downstairs and that, at most, the furthest I've been from the front door is the kitchen (from where I can see the front door) I find this somewhat hard to believe. Plus the engineer shouldn't have called before 8 am (the AM window is 8 am to 1 pm).

So I phone the fault line and they say the job is still open. They have the correct mobile number for me and I assume that the do know where the house is as they previously installed the line in the first place.

If there's a missed appointment due to no access, they should put a card through the door with the time of the attempted visit. If there's no card, then maybe he went to the wrong address

Hopefully he'll turn up and you'll be able to demonstrate the fault.

Good news.

The Openreach engineer turned up at about 10:30. He was surprised when I told him about the apparent missed appointment.
Sounds like there is a bug with the BT online service.

Anyway, he did indeed find a fault very quickly. The Master Socket was damaged. It was registering as if the phone was permanently off the hook.
So he fitted a new master socket and tested it. It seemed to be fine.
I've just had an incoming phone call and the broadband appears to have stayed up as well. I will keep an eye on things but it looks like it was just a faulty master socket (bit worrying, it was less than 18 months old).