Oh! Calamity

[#1]

Adam, is obvious that the internal cable is not to spec. CW1308 and from Colin's last post, it's not twisted pair. It is totally pointless raising a fault when that will be the main (if not sole) cause of the poor performance. This best dealt with here on the forum.
@Colin
That's fine, in your own time, no rush.

[#2]

@Adam,  Thanks for advice, will keep an eye on it. Current data is exactly the same as yesterday and is acceptable for now.
@A1.      Can I consider doing a replacement of the existing cable with that of the spec no. that you quote? It will be a significant grovelling job but not really beyond my ageing (ex electronic engineer) capabilities! I have found the leaflet that appears to be the one left by the installation engineer. It says it is NTE5 but does not look like the one illustrated by Old Jim in that there is no Test socket inside. The external socket appears to be a test one in that there are only 2 wires going to it on pins 2 & 5 which means that it is not suitable for a telephone (?). All the connections to the house extensions are wired in the base under another cover. This will be obvious when I get a pickkey taken. If I do fit a new cable, would it be suitable to connect it to the test socket or should I wire it in to pins 5 & 6 within the box?
The  plot thickens!
C.

[#3]

Hi CT,
Yes that is likely to be far and beyond the best thing, (it would be terminals 2 & 5) but before committing to do that, I think it would be a good idea to see some pics when you are ready, as it would be nice to check what's what and be sure that it should be fairly straight forward. I want to be certain there aren't wires disappearing into places we are not sure off  
So you don't think that this supposed NTE5 looks like this on the LHS with this test socket
By the way, you can leave your age out of it, as long as you still feel like a teenager  
When you do take pics, apart from the front view of sockets, need to see the wires and their connections to terminals.
Another (but different) C (in case you'd forgotten)  
Edit: When you start investigating/taking pics don't forget to use the disconnect procedure in outlined in reply #8

[#4]

That is encouraging. I have printed out your ref. which is handy for photos. At first sight the box does look like NTE5 on the outside but not the inside as you will see. Disconnecting drill very easy on Belkin N and will be used. If I do rewire I may have to fit a new socket on the wall in the computer room to feed only the computers, the existing one has a phone on it as well (via the filter.)  This presumes that the loft box socket is used but with only 2 wires connected.
More later... GP consultation  and some Armed Forces Service guph to prepare tomorrow, life is never dull when you retire!!
 

[#5]

Ok Colin, this question is for clarification as I'm now getting confused  This loft socket, is this the one in the garage roof or is it another?