I've temporarily shelved this. I've worked at it for a day and a half. It's certainly not any failing of the scope - the failings are all mine.
I've owned oscilloscopes since the 1970s, been a programmer for slightly longer and been working with various flavours of microcontrollers for over a decade, so it's not as if I haven't been around the block a few times.
Unfortunately, for me at least, the instructions in the manual don't provide any context. They tell you to choose a particular setting but don't explain why. There are also some differences between what I'm seeing on the screen - I don't have a "Bit Roll" softkey for example (possibly a later firmware?).
So I'll just blame my advancing years.
I dug to the very bottom of my equipment drawer and fished out a very ancient and dusty logic analyser. Connected it to the circuit and had a result in 15 minutes - worked first time. (Serves me right for not trying it first, I suppose.)
I'll definitely revisit this as I want to get it going so if you do achieve some success, please post it.
Thanks again for your valuable assistance.