Planning for FTTC upgrade: VDSL cable length?

[#1]

I have an upgade to FTTC scheduled for later this month and I am planning where to put the BT modem. Currently I run a 5m RJ11 "ADSL extension" cable from the filtered faceplate on my master socket, through a hole in an internal wall, to the ADSL router in the next room. I'd like to put the new router in the same location as the old one, and the the BT modem next to it. Can anyone tell me how long the standard BT VDSL cable between the master socket and BT modem is? Assuming it's not as long as the 4m I need, can I use my existing 5m RJ11 cable instead of the BT one? Otherwise I guess I will need to get the BT engineer to install the extension kit.

[#2]

Just use your ADSL extension cable as that is at least as good as the cable BT will provide.
BT provided me with a cable about 1.5m long with RJ11 plugs on the end. As far as I can tell it wasn't even twisted pair. I later replaced it with a 5m ADSL cable (also I think not twisted pair) without any degradation in performance. Although I have replaced that too now with one I made myself that is twisted pair, which also makes no difference to performance (for me).