OK the problem is resolved :-
1. My router was on its 59th day of uptime
2. I have only ever reset the router back to factory settings once, when there was a firmware upgrade.
3. Before doing a System Restore back to before the WinXP update on the 9 March, I decided to hard reset the router back to factory settings as I could not Telnet into the router.
4. Tone 89 is now at 14 bits and Tone 102 is now at 2 bits
So there you have it, perhaps it's good practice to resync or reset the router on a routine basis.
Edit : Routerstats and DMT now connect and run correctly. There was obviously something affecting port 23 rather than the telnet operation.
@jeffbb >>>>> your first post on page 1 of this thread, I believe, analysed the "problem" correctly.