Update
I downloaded Disk Warrior and transferred it to the external HD, then tried the repair, having booted the G4 from the firewire external drive.
It seemed to go fine until step 5, Locating Directory Data, when a message said 'speed inhibited due to disc malfunction'. Disk Warrior Techies very kindly advised me by email, to leave it running, maybe for hours/days!
I'm delighted to say that, after about 10 hours it worked! The G4 will now boot from the resident drive. So now I just need to get the important files/folders etc. off it onto our new G5, then I can install a new HD and get the G4 back to health again.
Many thanks for the kind help folks, much appreciated.