Went out for the afternoon - and it failed again.
This time I checked my router and it still had its WAN IP address allocated. After a reboot it said 0.0.0.0 so DHCP from the CM wasn't working.
DOCSIS 1.0 Cable Modem 60678EU <<HW_REV: 1.15 ; VENDOR: Ambit; BOOTR: 3.13.2 ; S
W_REV: 2.67.1011 ; MODEL: 60678EU >>
System up time = 1 days 00h 12m 16.00s
Ethernet single-collision errs.1 = 4731
Ethernet multiple-collision errs.1 = 3616
Ethernet deferred xmits.1 = 43865
Downstream channel ID = 0
Downstream channel frequency = 402750000 Hz
Downstream received signal power = -2.8 dBmV
Upstream channel ID = 3
Upstream channel frequency = 25584000 Hz
QoS max upstream bandwidth = 300000 bps
QoS max downstream bandwidth = 3072000 bps
SigQu: Signal to Noise Ratio = 34.1 dB
Cable modem status = Operational
Upstream transmit signal power = 56.0 dBmV
Date and Time = 2005-03-13,17:25:58.0
Configuration filename = cmreg-ntlhm200-high.cm
Aside - Docsdiag sometimes doesn't work as somehow I can't always access 192.168.100.1 behind my router. It doesn't seem to follow any pattern - sometime it pings OK, other times "destination unreachable" ...
I'm so used to rebooting my CM that I forgot to try out the coax disconnect/reconnect idea - must try to remember that for next time!