OK, bot has now been given a process management system to help run continually and not crash completely if there is an error. Not sure if it will help with the not posting at/after midnight issue, but it can't hurt and will give me a bit more information.
I have instructed the bot to post at 11:55, but after past failures am much less hopeful it will actually work โน๏ธ I've added in a few more test posts elsewhere at various intervals to try and get some more information about what is going wrong. At the moment the theory that the cat is sabotaging it seems as likely as anything else. It is very frustrating that every single test I do works perfectly, and the actual post doesn't.