As I understand it, you have an ADSL/Modem (
Access Point A) connected to O2 at one end of the room and an ex-Virgin device (
Access Point B) at the other end of the room. It is (was) possible to connect to both boxes over a wireless connection.
You want
Access Point B to talk to
Access Point A using a wireless rather than a wired connection and then to be able to plug a PC (or laptop) into an RJ45 port on
Access Point B - is that correct? I believe that this is called wireless bridging - have a look
HERE.
For this to be able to happen,
Access Point B must be able to establish itself as a client of
Access Point A. Like you, I think that this ought to be routinely possible - sadly however, it seems not to be
It may be that if you have the right devices - i.e. Linksys (doubtful with ISP supplied kit), some of the modified firmware such as
DD-WRT or
Tomato may make this possible. Perhaps someone here familiar with this firmware can comment?