Development tools for Zephyr on STM32Workbench for Zephyr: VS Code extension to set up, build, flash and debug Zephyr applications on STM32.Devicetree manager: browse, edit and visualize Zephyr devicetree, STM32CubeMX-like for Zephyr.
I checked the probe interface and found that the NRST signal was not getting to the target. A case of bad soldering. All working now as far as I can tell. Many thanks for your help.