After restarting the machine I have found:-

1) The file that became 'corrupted' is still protected
2) I do not have access to the 'styles menu' in that document - all the styles are greyed out in the ribbon
3) It is not obvious how to remove the protection without access to 'styles'

Using version history, I have recovered a earlier version of the document, and have been able to insert the table and caption after saving it as a new file.

My observations is that once the problem occurs in word - it persists (100% failure rate) until the the machine is restarted.

The problem with intermittent problems that you can't identify the cause when the fault does not occur.

Thanks for you help and interest