I had a moment of confusion with broken Insert Options recently, which made me wonder whether I ever knew the answer behind the issue or whether I'd just forgotten it over the many years I've been working with this CMS. Either way, this needs writing down to help me remember it in the future...
There's quite a bit of material out on the internet about creating custom fields in Sitecore. But inevitably, when I found myself with a requirement to create one, I couldn't find anything particularly close to my needs. So here's another post about creating a custom field – maybe it'll be of help to someone...