Just wanted to let anyone interested know that I had this problem. Planet advised me to reflash. I did so, but reluctant to wipe everything I'd set up, I only reflashed some of the partitions: preloader, recovery, md1img, md1dsp, md1arm7, md3img, scp1, scp2, lk, lk2, logo, tee1, tee2, and boot3 (my android boot partition). It looks as though that has worked, though I'm waiting for the unit to charge a little before restarting it and checking that I've not lost any data. It started rebooting successfully into Sailfish but then shut down 'cos of lack of power before I could do those checks.