"Your advice it to get a seperate peice of equipment
which acts as both router and modem "
slight missunderstanding there it seems, and noone bothered to put it right all this time
its NOT a single unit that replaces both the current cable modem and the (wireless) router, its just replacing the old internal docsis STB modem with an external one, and replacing the crippled NTL/tw/VM supplyed router with a generic router than may be better for you.
Cruachan's advice seems sound so you could go with that option....
weather you use the internal STB VM-CM(Virgin Media set top box Docsis modem) or a new seperate VM-CM doesnt really matter, they are just trying to get everyone off the STB version so they can save cashflow running two billing versions.
the STB version gets billed on your current TV package bill, the new external one gets you a seperate bill that doesnt have a bar code, and you need a direct debit setup for that new BB account.
so whatever you use, assuming you never intend going above the 4Mbit/s BB package, then you will always connect your wireless routers WAN port to that CM LAN rj45 cable connector.
they will just fit or have you fit a supplyed cable splitter ( and that may mess up your signal levels if they dont bother to check at the time of fitting) one end going to the current STB and the other to the new stand alone CM.
then if they havent entered and activated the new CMs MAC into the VM system then you may find you loose BB until its sorted and entered properly.
if you replace the current working wireless router , then you will need to get the new routers MAC address registered with the VM server side kit, that can be easy or hard depending on what kit and firmware is inside it and what part of the VM network you happen to be in.
you can take the old routers MAC and clone that into any new wireless kit if you have problems registering the new one, though it sounds like your new to all this networking stuff so it might be a hard learning curve for you!
on a side note if you intend going to the current 20Mbit/s or the future 50Mbit/s
and want wireless use rather than wired use , then you would be wise to look at the new 11N MIMO kit that can give the required full throughput for this increased web speed... dont forget to buy 11n kit for both ends.
that includes all PCs on your LAN side that you want as wireless connections, mixing old 11g/11b and new 11N kit does nothing but slow the LAN connection down to the lowest kits speeds and thats a waste...