Apologies for those who were faced with an 'Unsecure Site' notice when trying to log in. It turns out the site's SSL certificate had expired.
Here is the explanation from our site host...
"Hi Brian
Sorted: it seems the SSL certificate had expired. It normally is reissued automatically for a period of 3 months and I don't know what went wrong there, but it's now fine.
I'll keep an eye on it next time but don't expect a problem. The cert expires on 24 Nov 2018 and I'll add a note to my calendar to check on that date.
Best wishes
Martin"
So, it wasn't hackers or aliens after all