OK. I finally worked it out. For anyone interested, here is the process.
Remove the battery and WAN card cover.
Remove the central screw in the bottom cover (marked with KB/HDD symbols).
Locate the thin slot on the upper right hand side of the keyboard. Insert a blade in this slot and lever up gently.
There are 4 tabs retaining the top of the keyboard. With a thin bladed screwdriver, press down to release, starting at the right hand side. The keyboard can now be lifted up to reveal the ribbon cable which must be disconnected. Pull out the brown retaining tab 1 or 2mm first.
Remove the remaining 6 screws from the bottom cover and the retaining screw under the WAN card cover.
Unplug the small ribbon cable leading to the finger sensing pad.
Remove the screws fixing the top cover - these are all labelled M2 x 5 or M2 x 2.5. The top cover can now be removed by inserting a spudger or similar tool between top and bottom cover and prising apart. I found it best to start at a front corner.
Remove the screws locating the circuit board to the right of the motherboard and fold the board up out of the way. It is not necessary to disconnect it.
The screw locating the metal HDD carrier can now be removed and the carrier lifted up so that the HDD itself can be removed.
Reassembly is the reverse of the above.
I hope this helps somebody!