Well it connected and synched at first attempt but otherwise a disaster.....
A TBB speed test showed loss of 3Mb/s down and 0.6 up. However, bigger problem was that of the 5 terminals available here, every one failed to connect to the internet except for the one I ran the speed test on (and they all go through the same Cisco switch) . All adaptors have always had 192.168.1.254 as the gateway and as the ZyXel now has that as its IP it should have worked out of the box.
Even worse, the MDWS server was failing to pick up anything incoming. I tried running the MDWS web page locally on it which should not be any problem - and it went to
www.mydslwebstats.co.uk - but showing the ZyXel logon page
I can't spend any more time on it at the moment but ideas would be welcome..... Maybe a misconfig on the Zyxel?
Back on HG612/582n at the moment and all is normal