ReadByte not working in tutorials/SerialMonitor

[William-Purvis]

I've just installed tinygo and tested the blinky and serialmonitor tutorials. The basic serialmonitor code that simply writes "hello World!" works fine, but compiling the more advanced one which accepts keystrokes over the USB port waits for input then simply
types 'p' and then hangs. I've tried putting extra prints in but nothing useful shows up.
A print before the call to ReadByte works - generates a flood of prints until I type something in, then responds with a single 'p' then hangs, no further output.

I'm talking to a waveshare-rp2040-zero.

Any ideas welcome...