Thanks, Anotherone - appreciate your suggestions. And yes, five BT engineers have indeed visited and diligently repeated precisely the same tests as the previous engineer, and come to the same conclusion ie. their tests cannot find any problems. One engineer did suggest that the line card in the local exchange might be at fault, but I didn't hear any more about that and I don't think this was fed back from BT to PN (I did add the info to the support ticket, though). Another engineer suggested that a line swap might do the trick, but he said he wasn't allowed to do that anymore (whatever that means - he clammed up after that...). But all engineers managed to get a synch rate of about 6000kbps when conducting tests with their laptops; unfortunately, this synch rate degraded a few hours after they left and they were therefore unable to see the problem for themselves. because of this, I've told PN that I don't want any more BT engineers wasting my time with repeating tests that are unlikely to succeed in diagnosing the problem.
I've replaced my DECT phone with a corded phone and I'll leave it for a few days to see if that makes any difference. I'm currently using an ADSLNation XTE-2005 filtered faceplate, but I previously used one of their dangly XF1e filters (I swapped filters to see if this was causing the problem, but I don't believe either was at fault). The XTE-2005 is effectively plugged directly into the test socket; prior to that, the XF-1e was also plugged directly into the test socket i.e. I removed the top half of the standard BT NTE-5 faceplate to attach both microfilters. No extension leads were used for filters or phone or router.
I've not noticed any additional noise on the phone line around the time that the ADSL connection is dropped, but that's not to say that's always the case (as many of the disconnections happen when I'm away at work).
After the BT phone line comes down to my house from the telegraph pole, it is fed into a small (approx 6x6cm) white BT junction box just under the eaves - the wire then disappears into the roof space, before exiting on the opposite side of the house and running down the wall before connecting directly into the master socket. The reason for the roundabout route is that I asked BT to move the master socket to another part of the house just after I moved in (about 18 months ago). At least three of the engineers have tested the line quality at both the master socket and the junction box under the eaves, so I guess that's covered that potential source.
I live in a semi-rural area and there are no organisations (police, taxi, electricity board etc) within a mile or two at least. The nearest mobile phone mast is a couple of miles away, and the closest electricity substation is about a mile away. Gatwick Airport is about 3 miles away, as the plane files. I can't see any obvious signs of unusual radio masts nearby that would indicate the presence of radio hams etc. The nearest TV transmitter is about 7 miles away. My TV aerial is just a simple terrestrial type i.e. no freeview or satellite, and no signal boosters either.
I'm more than happy to continue checking for potential interference sources in and around my property, but my "gut instinct" is that the problem is caused by something in the local BT infrastructure i.e. faulty connections, dodgy line card etc. Is it possible to get BT to do any more advanced checks on the line (remotely or at my property) as the standard checks are plainly not hitting the mark? Or is it possible for PN to ask BT to consider replacing things like line cards just to see if this resolves the problem? After six months (going on seven), I can't believe that PN and BT are happy to continue sending out engineers (at about £200 a visit) when the money could be better invested in an alternative strategy, perhaps.
And thanks to everyone who has made suggestions, however frivolous, as one of them may just help someone think of a way to resolve this very, very long-running problem.
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