[Halcyon]

Changing the length of cable or adding your own cable anywhere on modem side back to the wall is not a good idea. You can change your signal levels and also cause problems to othe people on the network.
I'm not saying it isnt possible, but that it cancause problems if not done correctly.

That is why I reccomend that if you can leave the modem next to your wall point do so and instead use a long ethernet cable, or indeed wireless access point if desired to connect from modem to your PC.

EDIT:

Ok, in that case you will need to have the coax cable extended. If you see a Virgin engineer down your street, bake him a sandwhich, give him £20 and he may do it for you. You never know.