Problem solved (after a fashion).
One of the reasons I prefer the 3com router is that it is very flexible and provides reasonably detailed logs.
On the 3com router I had set-up the local network IP range to be in the 172.27.xxx.xxx block (perfectly legit - see http://en.wikipedia.org/wiki/Private_network). This has always worked fine before.
On the Thomson (as supplied in a pre-set and locked state by my ISP) one cannot change the local net range and so are stuck with 192.168.xxx.xxx
Changing the 3com router to be in the 192.168.xxx.xxx seems to have overcome the issue.