Just as some feedback to finalise this issue, it's been running for 36 hours or so logging every 15 minutes without error, so that does seem to have been the problem.
I guess I should replace the other cron entry to make sure it raises the errors again, but I'm more focused on just finishing the project for now. I still need to get ssh working from outside my lan (router firewall issue), sort out voltage and cpu temperature monitoring so I can log it, replace the bios battery in the MB so it keeps it's date in a power failure, source a silent power supply, source or fabricate a case...