Regular disconnects

[#1]

This isn't a question of money, even if the wiring was covered by any warranty, they won't fix the broadband problem, they will only ensure that the wiring is CW1308 and wired to the correct standard. That would include the bell-wire which will be of no help as previously explained.
Unless the wiring was damaged on installation, there is little point in trying to involve anyone else to resolve this rather trivial problem, and from what the OP has posted, it's well within his capabilities. It will no doubt be quicker, easier and less hassle to resolve it himself.

[#2]

very true,
sounds rather like his building firm used the same electrical cowboys my landlord's did, but its not about the triviality of pushing the wires into the idc connectors (just push them down and see if it fixes the problem) but if the problem doesn't go away about who's paying for a wiring test on a faulty installation, the cowboys that fitted my place out used sealant to fix all the face plates to the wall so opening them invalidates the warranty ....
also if they fail to comply with the fault rectification details listed in the owners hand book for the property you have financial recourse to claim some Wonga off them for breaking the contract......

end of the day if it was me I would just isolate all the sockets I wasn't using and see what happened, but only If I was sure it couldn't be argued about by them...

[#3]

This is undoubtedly a bell-wire issue which has been present for a while and recent local work has exacerbated the problem to the point it's now noticeably affecting the connection on a frequent basis.

[#4]

Thanks all for the really helpful advice.
Have checked the wiring and replaced the three internal sockets.  Corrected the wiring as suggested, by putting both sets into the socket rather than twisting the wires together (as was).  Removed bell wire too.
Also two sockets did have two sets  of wires going into them (photo didn't show this clearly), so line went upstairs first, then down to hall then into living room.
But all sorted now, speeds increased to 7.5mb from previous average of around 5.5, all down to dodgy wiring.
Connection been stable for hours now... Here's hoping it stays like this.
Thanks to all again.
JWH

[#5]

Let's hope that continues for you and no more hassle