Went back to Maplin Electronics yesterday and got an Mapower enclosure. It’s new so isn’t in the catalogue yet but it has an Oxford 911 chipset for the firewire side of things. It works with the mac just fine and even lets you boot from it using Alt to select a boot disk.
It takes a standard 3.5″ hard drive (there was a 200GB on sale for £80) and converts PATA into firewire400 or USB2.0, both of which will give a pretty reasonable throughput. This is what it looks like…
They also do cases for 2.5″ laptop disks and for 5.25″ CD/DVD drives.