RESOLVED: Creating a new device with a new name seems to have avoided the error being thrown.
When I check the Devices / mydevice / callback Authorisation checkbox so as to get a permanent Access Token I am getting the following error message…
Ooops! Cannot process your request. Error 400 (failed to validate request schema)
Everything worked fine using Postman with the non-persistent token and this device was updated by the json as expected.
Any help and advice gratefully received.