Ok here is a summary of what I have done at my girlfriends over the weekend, initially I unplugged the new handset base and unplugged it from the splitter. The line stats didnt improve. I then started with the extension socket in her room and only left wires 2 and 5 connected, no improvement. I then went into the other room where there was another extension, this extension is fed from the master socket in the lounge, and the wire for the upstairs socket is also on this faceplace, I did the same thing and disconnected all the wires leaving 2 and 5 connected. No improvement. Lastly I went into the lounge where the master socket is, disconnected all cables except 2 and 5, no improvement again. Amongst resetting the router over the course of this hour or so time, the SNR did go back up to about 7.5DB and connected at 4096 like previously but there was no reason for this at all. So I think we can now rule out the new phones, but the weird thing I dont understand is why is this now a problem

I plugged the router into the master socket and it was the same, I was a little naughty and did check the wiring was all fine at the back of the faceplate, which it was.
There is a couple of other things though, but I dont think they are causing the problem as like I say everything was fine till the new handsets came. In the hallway there is one of those old fashioned bell ringers for people hard of hearing so they know the phone is ringing, i dont know how you would disconnect this but I dont think that is a problem. The cable outside enters the house just next to the main door, there is a little white box, it just has wiring inside, looks fine to me though.
May it be worth giving the connection several days to see if anything improves and if it doesnt, contact Sky? another thing which seems odd, is that their upload rate is very low, 415kbps.
Here are the line stats, SNR currently at 100%:
ADSL Link Downstream Upstream
Connection Speed 3092 kbps 415 kbps
Line Attenuation 54.0 db 33.7 db
Noise Margin 7.4 db 10.4 db
Here are my line stats just to compare (my SNR margin is manually set at 1% and my connection is just as stable as when it is set at 100% lol):
ADSL Link Downstream Upstream
Connection Speed 3936 kbps 768 kbps
Line Attenuation 57.0 db 31.5 db
Noise Margin 6.3 db 13.0 db