Thanks for the comments guys. sgk is correct - I don't often use the wi-fi bit, just 2 pc's wired to router (ZyXEL NBG334). So I wouldn't see rogue traffic on my network, it would just steal my bandwidth. Occasionally we linked a Wii console via wi-fi,so it was left on. Our password was generated by the router - random alphanumerics. With wi-fi turned off, nternet still 100%. We are not using 50Mbps or Super Hub, just standard 10Mbps.
A guy at work had this problem- he lives in an area in Sale popular with students. He is with Zen Internet and pays for his data. All his allocation was used so he paid for 1G more, to find that used too in 2 days. All he'd done was a bit of surfing.
I am very surprised that this is the problem but if you google "wifi hacking wpa2" there are lots of sites that claim to help you do it.
Anyway will keep investigating...