Solution - Different Model Burner
Went back to Adelong Computers. They were happy to exchange the drive for another, but had only one other IDE burner, which was an LG GSA-H55N.
Took it home, installed it into the enclosure and bliss! It burns DVDs from the Finder, from iTunes and via Burn program. Disk Utility still reports the drive as burn unsupported, but nevertheless the drive appears solid for single layer DVD-Rs, and is much quieter and runs much cooler than the crappy internal Suprdrive in the iMac.
I'm very happy with the service I received from Adelong and with the nifty new Alu enclosure which I have on my desktop.
I can't recommend using an LG GH22NP20 burner; it appears to be not fully functional under the current OS X 10.5.7.
Simon
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