Being impatient, I have tried to answer my own question and it looks like it has worked.
Basically I did a factory reset. Note the method is slightly different for all phones, so I used Google search for Moto G. The smart phone appeared to be working after a few minutes. However it soon became clear that many google apps didn't work as 'google play services' needed an update which I couldn't do as my phone was too old (android 5.1). However the old smart phone was cleverer than I thought and automatically downloaded the upgrades for numerous apps over the next 3 hours or so (so the list of 'downloaded apps' (in settings/Apps) grew with time). Then most things were working again.