OK, so I finally seem to have fixed it myself (thanks to a post on Reddit).
- Ensure you have BIOS version 1.2.3 or later; if not, update it
- Shut down computer
- Start up again while holding down F2, to go into the BIOS
- From the menu choose Power Management, then Block Sleep, then check the Block Sleep box at the top
This removes the Sleep option from Start menu>Power (and elsewhere), but fixes the problem. From the behaviour, it now seems to be hibernating rather than sleeping when I close the lid or press the power button, in that it takes longer to 'sleep' and wake up. Whatever, it works.