Glad you managed to get it sorted Hugh.

Yes, you need that confirmation email, otherwise I think you should assume you will be charged full price next month.
I would email FindMyPast help and tell them (clearly) that you do NOT want to autorenew and you are unable to get the box to stay ticked. See if they can sort it out for you rather than hope for the best in a month's time.
They are usually good with getting back to you, though sometimes I think they get a bit confused, which is why I advised making it very clear what you want.

(I've had to re confirm this kind of thing with a couple if different people in the past)
Good luck.