Update on this issue. Greenbank OTP kindly sent/leant me his two original G5 discs + a copy of disk 1.
For some reason Firewiring up the monitorless G5 tower in Target mode with the G4 iBook - it shows up both on the G4 desktop and in Disk Utility. BUT via DU it won't erase. It will Verify and Repair Permissions but the rename and Erase options are greyed out. This applies to both the main Firewire icon and the indented named 'PW5' Firewire icon.
I inserted Greenbank's Install disk 1 and it's lablelled 10.4.2, wondering if I could erase the HD with that and then reinstall - but without joy. The OS on my G5 is 10.5.2 - I have no idea if this is of any consequence?
It seems second hand 160Gb and 320Gb used HDs are available on eBay for £25-£30. Sadly the separate 160Gb Samsung HD I previously used as a backup has totally different connections and isn't compatible with the G5.
I'll see if Efrogwr is still interested or it's be down the municipal tip.
The G3 blueberry fires up fine, and click calcks away a good 'un as it does its thing - everything seems working