I had another stab at this and managed to get the darn thing open this time. The circuit board (and internals) are connected to the (top and side) metal casing by 4 plastic pins. Using a flat screwdriver I was able to prise them out of the casing. Then I could lift the drive internals out. A careful dusting down has brought the drive back to life.

Here's a pic to try and illustrate where they are. Wedging a screwdriver between the black plastic above the circuit board and the metal flap allowed me to me pop the plastic pins on the four corners, separating the drive from the outer casing.