Thanks for this advice.
(and thanks to MrsBG for letting me use her iMac to contact you all whilst my pride and joy is under discussion!)
I've re-set the SMC and it has re-established the connection to my Time Machine on the FW800 port. Whether doing this, or just removing the battery as suggested is the solution I don't know.
It's spending quite a while 'cleaning up' but at least there's activity and lights flashing which proves the port is still working.
The other drive on the FW400 port (which was only a clone of my main HDD) is still not responding. But I'm very relieved to have the TM back, I can tell you!!
(EDIT: what's happening now is: all's OK with Time Machine until I plug the other drive in ... when I lose both with a 'you didn't shut down properly... etc. Unplugging the FW400 drive and re-booting gets the TM back. looks like I've a prob with the old FW drive/connection or both)
Edited by biffs (Sat 04-Jun-11 10:03:59)