No problem!
I've got further with this now - as described above I had to add the keyboard app identifier to the system apps profile but it still wasn't working so I then added the specific GBoard app tag to the list of tags against each device and then it downloaded and installed GBoard.
Then I went into the settings on each device, into languages, keyboards, virtual keyboards, manage keyboards and could then add the GBoard keyboard. Once that had been added I could then enable the GBoard keyboard which finally allowed me to disable Google voice typing.
Now I finally have an onscreen keyboard available again. I have updated the ticket with Meraki to ask why the Android Keyboard has suddenly been removed but had no response as of yet.
The GBoard settings may need refining to suit your environment/needs - for example I removed the emoji keys, but at least it gets a working keyboard up and running again.
Alex.