Sorted! (sort of).
A colleague in our IT dept got interested and suggested that burning FROM a USB device TO a USB device was not a happy match. I copied the source ISO file to the internal HDD and cut the speed of writing to 8x as suggested. Worked like a charm. It's not a happy result because our archive is all on 1TB external drives and now every time I need to burn a disc the file has to first be copied to the HDD. Still, it now works.
For the record though, and to help with future debugging:
AMD Athlon II P320 dual-core, 2.10Ghz, 3GB RAM
Sumvision Tempest enclosure (cheap and cheerful I know, and a pig to put together, but seems ok, I know a few people who have them)
Windows Home Premium x64
Thanks for all your help, and Kudos for ImgBurn - a brilliant piece of work in my opinion. Matched by support second-to-none in the business. In the days when I suffered under Nero I would have _killed_ for support half as good as this.
LongSoft
"Satisfied in Southampton"