Thanks for replying Kirk. No - the opposite: what was happening was that previous templates were changing when I didn't want them to. But I just tried again and it worked fine. I must have been doing something wrong.
But here's a new but related problem: if I change a template, then go to close stationery, it asks me if I want to save the changes, but if I re-load the same template, the changes haven't been saved. How do you edit an existing template without changing the name? (I"m going to post this as a new question aswell).
Thanks for your help with this.