Hi there and

to the site.
It is not advised to change the length of the cable that hooks upto your NTL outside box and your modem as it can affect your signal quality.
If you need to just disconnect it and then reconnect it though, that shouldnt be a problem. Just make sure you power everything off first.
You could always just leave the modem where it is and buy a length of Ethernet cable to go from the modem to your PC.
I bought a 25 metre cable that I have running under floor boards and it works fine.