Owning your own home

So this is only about the initial purchase, right? Not if someone moves in later? Would this apply again during remortgaging?

Yes, no, yes.

I'm a bit unsure what this means, I get that I can't rent it out, but other than that - when are third party rights of occupation acquired? Only through eg a tenancy contract, or just by someone living there?

It's a bit unclear as it's trying to be user-friendly and so uses non-legal language. A Significant Other living with you wouldn't have a tenancy, but merely a bare licence which gives them no right to occupy the property without your consent. I doubt a bare licence would count as a 'third party right of occupation' on any sensible reading of the mortgage. I think you'd be fine.

Anyway, the real reason mortgagees include that provision is so that if you do grant a tenancy or give someone else an interest in the property, they're not bound by it. They don't really care if you do or not - they just want to make sure that they're not bound by any arrangements you make. And telling you that you can't do it provides them with that protection.