Good thought.
I grabbed a wifi analyser (WiFi Scanner, by Lizard Systems - hooray for trial periods that cover my needs) to look into that specifically. It’s on channel 6, and was running at a bandwidth of 40 MHz; I’ve reconfigured it to 20 MHz. Channel 6 is relatively uncongested - there’s a few networks from neighbours, as you’d expect, but channel 6 is the one showing the least sign of competing traffic.