Okay just to update on this.

After a lot of fiddling around moving bits between the laptops trying to figure out what's what, the upshot is that the hard drive in the Powerbook is knackered.

Not sure why it wasn't seeing the DVD drive, can only assume a bad connection, because it's now seeing it okay, but still won't recognise the hard disk. However, when the HD is swopped in from the working Powerbook, it boots the machine fine. Only conclusion can be then, that it needs a new hard drive.

Many thanks to all those who contributed.