If you have WiFiKiller installed disable it before updating. Just to be on the safe side..

Updated Xperia X this morning. First attempt to update failed
On trying again I disabled WiFiKiller and it only downloaded a few bytes then went to update OK after that.

Just updating my J1 now. Getting "Not enough memory to apply update" errors on that. Haven't got many apps, images or videos on the device either. If it fails again I may try via terminal.
This has now got me thinking and asking myself 'Is Sailfish now getting a bit too big for the original Jolla 1 device or is it just a case of too much left over junk from previous installs, apps and temporary files clogging the system?"