Morning all. From the logs, the reason for the server disappearing is that 'the peer' (ie the ISP) is failing to respond to echo requests. The actual reason I don't know - the ISP not responding, the line dropping, etc. I've extended the interval between echo requests and increased the number of attempts, which may help. It was already set to immediately re-attempt connection up to 25 times, which it appears to not be doing, annoyingly, but I've changed that to 'unlimited' anyway.
Hm. The outage over the weekend may have been caused by a massive outage in BT's Manchester centre, which it seems is rather a large single point of failure.