My apologies for the site being unavailable yesterday 20 April 2017. ( Afternoon and Evening UK time). There was an outage in the data centre that hosts this server. The reason given to me was a firewall cluster error or failure. This resulted in name resolution problems, so nobody could access the server except directly via IP number which of course gave certificate errors. My DNS servers are all in the same data centre.
I have now added extra DNS servers at the registrar level, so hopefully any future DNS resolution outages should be alleviated.