Managed to get the Master Socket moved over the weekend...
Some pictures of the before - the blank conduit is for mains cable for the Air Source Heat Pump, and was installed after the BT Drop wire. The poor quality crimping on the cat5e adsl wire was my doing when I had to move the Router in a hurry due to new radiators being installed at the same time as the ASHP (but it worked fine and I never got round to replacing).
8mm x 400mm drill bit, went through mortar outside and made a reasonably clean hole on the inside. Secured the new backbox with 4 screws as 2 didn't seem very sturdy.
Removed the MK3 faceplate from the old master socket, and then the front of the socket to find... "Jelly Crimps". Not sure why they were there, but certainly not going to beneficial. (Also the cabling doesn't follow BT's "standard" as everything I read says that orange should go to "A")
Cut the cable where it originally went through the wall, then undid all the existing cable clips and pulled it back and rerun down the wall to the new hole. After feeding through needed to cut approximately 2 Meters off.
Could probably have trimmed a bit more off as there isn't much room behind the faceplate, but the new camlocks are easy - just insert the Orange and Orange/white wires and then press down to make the connection (and I wired them the "correct" way).
Master socket now within 1m of the router, so back to a standard premade DSL cable (although I have ordered a couple of shorter ones to try).
Initial impressions on Saturday were good - Upstream synced 250Kbps higher straight away, Downstream showed an attainable rate of around 34Mbps (although still connected at 22399 as I believe the line has been banded).
However upon monitoring for most of yesterday the Downstream attainable and SNR Margin have been all over the place (dropping as low as 15999, although the line has stayed synced at 22399 and not dropped connection) - not sure what's going on with this