Following your advice, I deleted (within the Boxes software) each of the devices that were "Live" but reported as "Inactive". This seemed to clear the problem.
Unfortunately, that didn't last very long!!
To cut a long story short, the "connected" devices were no longer being reported by the Box (either as "Inactive" or "Active".
However, I did notice that the "Clients" were now being allocated IPs in the 169.254.0.x range (should be in range of 192.168.1.x).
After a long, long, long phone-call to O2 support, they have agreed to supply me with a replacement Box.
btw, I did try the DG834GT, but was surprised that it significantly reduced my d/l speed. With O2 Box I'm getting 18+Meg, with DG834GT I'm getting 15Meg.
Regards,
John