Thank you for your response and reaching out with help. I have worked around my issue by starting the project from scratch and therefore no longer need the file. I'm happy to help if the file could offer a hint to the cause of my issue and by such preventing it from occurring again.
As for the additional information:
I'm using Windows 10 20H2.
The file was worked on and saved in Affinity Publisher 1.9.0 unfortunately I don't have a record of the remaining part of release number as an updated of the program has come out since.
It was saved on local SSD, separate to the SSD which has the system installed on.