...the disk sold by Bentham Ltd was defective! This may be a one-off or a bad batch, so for the benefit of those who may be suffering read/write errors, poor data transfer rates and bad sectors, I suggest you download and run the rather useful SeaTools from Seagate's warranty section.
A word of warning to those unfortunate enough to have a disc generating an error code... Bentham Ltd require you to go through Seagate's RMA procedure rather than supply you with a replacement. This means returning it at your own expense and waiting 7 - 10 days... but first you'll need to read a 20 page packaging PDF (and no, bubble wrap will not suffice - so keep the original packaging!).