Well, whatever the issue was, it has now been resolved (thanks Tony). I see that the server status page has finally been updated to reflect the issue albeit many hours after the problems started to happen. I assume it is a manual update process which is only really of value retrospectively.
Irritatingly, there is still no explanation of what happened and no answers to the specific questions I asked in my ticket.
OOPS spoke too soon - server load back up to 300 and error 503 - inaccessible... Intermittent problem