Thanks for the quick reply. Since my original post I got it working with a phone charger, but it went off when I touched it. After a couple of attempts it loaded again successfully and I've left it on since then. The breadboard I'm using is 30 years old and hasn't been used recently, so I put this down to poor connections.
I get similar a response when I connect to the computer. I'm using the Chrome app on Ubuntu 20.04, so that could account for the difference.
The display on the charger does not work as described though. The bottom two lines remain as zero, though they went to dashes with one set of batteries I tried.
When I flashed the Pico it gave me three options for the firmware to install. I chose the top one with everything. Could that make a difference?