Of course, 10 minutes after posting I found the solution…
The hibernation file is set to ‘reduced’ by default (possibly on devices with little memory), so that only ‘fast startup’ is available, but not ‘hibernate’
The solution is to open a command prompt as administrator, and type
powercfg /h /type full
For me that increased the size from 400MB for the reduced hiberfil.sys to 800MB for full hibernate support. Still quite acceptable given the benefits.
