Thanks. It occurred to me that I had never specified a password for access to the modem, having had it for several years. I vaguely remembered seeing the 'Thomson SpeedTouch ST536' page in my browser - a long while ago - but couldn't even remember how to get to it! After a bit of faffing around I found that it was wide open. No password needed.
It now has a decent password.
I'll check out the telnet option when I feel brave enough. I'm a novice at CLI matters.
ZeN 8000 Active
iMac 20" 2.4 GHz Intel Core 2 Duo : OS 10.6.8 : iMac 15" 800 MHz G4 : OS 10.4.11 : PowerBook 17" 1.33 GHz G4 : OS 10.4.11 : Home build 3.4 GHz Intel Core i7 2600k : Ubuntu 12.04.4 : SpeedTouch 536 v6 : AirPort Extreme Base Station