Thank you so much for your reply, I was beginning to think it was perhaps a silly question.
I don't recall uninstalling previous versions in the past when I have upgraded, but perhaps I did, because when I upgraded from PD16 to 18 this time, the custom title templates are all still there!! The whole process was different this time, so maybe that had something to do with it.
However, perhaps someone could help me further - these custom title templates are still under a folder called 16 on my C drive under documents (presumably PD18 has a way of finding them), but I'm worried about leaving them there for future updates. If I move them into the new folder called '18', will my current PD still find them there? Or will it mess things up?!!
Thank you for the tip about templates on DZ - I always assumed once you uploaded, there would be no way of removing them later.