Paul
Try sending a PM to Andyf97 on this forum. He's done lots of wireless networking with the QM2000.Net cards.
The solution he uses is to use VERY powerful and robust transmitters that basically obliterate any interference on the frequency.
I haven't got access to my PC at the moment otherwise i'd send you the links to the gear. It's mega stuff, but it doesn't come cheap!
Cheers
Jem
Quote: "There is a theory which states that if ever, for any reason, anyone discovers what exactly the Universe is for and why it is here it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another that states that this has already happened.”... Douglas Adams 1952 - 2001