Hmmm, not as successful as I hoped
The first piece of film moved and got in the way of the darkslide, preventing me from replacing it.
The second piece, after development, has no trace of an image on it but is uniformly dark all over apart from the edges which are clear where it was held under the lip of the film sheath. I'm guessing the film holder isn't light-tight.
On the plus side, my development routine seems to work and I found it easier to work in total darkness than I was expecting.