Many thanks for your input.
My connection settings included the required Authorization and Accept headers but for some reason the connection was failing, which was why I resorted to trying the form at https://developer.companieshouse.gov.uk/document/docs/document/id/content/fetchDocument.html
Anyway I have subsequently tweaked my code and it now works, but have since discovered the document fetch process consists of two-stages. The first stage is the call to "document/FIxRR8teCKodjkBLRDHv2Cb8y0-nQ7T5G3BEXfWtOu4/content" (as per your example and with the correct request headers) which returns a HTTP response "Location" header, the value of which is the document download URL location. The second stage requires making a connection to that URL to retrieve the actual document.
Returning to the online form I can now take a stab at what input is required (although I could not get it to work ):
id field : document/FIxRR8teCKodjkBLRDHv2Cb8y0-nQ7T5G3BEXfWtOu4
authorisation: Basic T3FKdC1GekxLNHU2ePlusRestOfEncodedKey
I understand the service is still Beta but hopefully the documentation will be improved to remove the guesswork and confusion.
Once again, thanks.