It looks like the serial port locks up after I send the initial command using the serial monitor. The contents of two of the variables I asked for in the BP are displayed once only, then the output window ceases to scroll too.
Thanks for any help,
Paul

You should not need to be inspecting vm debug messages.

There are a few things to note.

1) When debug is ON, The serial send by your own code must be be terminated by a println() prior to Visual Micro sending it's own serial packets.

2) If the setup() method of your chip is called very quickly (starts quickly) there is possibility that the initial "debug hello" packet will be missed by the pc because they arrive before the serial monitor can open/start. This is not an issue but breakpoints and trace points in setup() can also be missed for this reason.

3) Sharing a serial port between debugger and your own code can be problematic if your own code does not send println() type terminator and also if your own code reads the serial port. In this case debug supports an alternative serial port if your hardware does or support a SoftwareSerial type of connection on one or two digital pins.

Great thanks.

Yes you can either use another serial port for the visual micro debugger or use a usb>serial cable along with a couple of spare digital pins. To use a different serial port you set vMicro>Debugger> "Local Port" to the other COM port and "Remote Port" to Serial2 or other valid port name.

Docs here 

Debugging-With-Different-Ports