I'm hoping to try some showing with my pup when she's old enough.

I haven't ever entered a breed show before but imagine an open show would be the place to start?

Where do I find a list of open shows? I can find the Championship ones but think these might be a bit much to start with.

Probably the best place to start is by attending ringcraft. 
Along with the obvious advantage of getting you and your puppy used to what is required, and meeting lots of new friends :), you will generally find entry forms available for any upcoming local shows.

I've found a local ringcraft class which runs every two weeks, there are no contact details so assume I should just turn up?

Yes, just turn up and for the first week it may be best just to sit and watch, the ringcraft will teach your pup to be handled but more to the point it will teach you how to show a dog.

Where you start does not really matter and a breed club would be a good choice as your breeder may be there to help you and their should be lots of support but after the breed club show I think I may go for Championship show as there will be more of your breed and they are easier to understand what is happening. Open shows can be more difficult to understand where you should be and what you should be doing but once you have been to a ringcraft class you will meet people who may well mentor you through your first few shows.