#N/A” Error Encountered When Copying Formulas in O365 Excel 2016

Hi kersplash,
Thanks for your prompt response.
I use the workbook for my financial planning and so it contains shedloads of sensitive data but I have extracted a few sheets and sanitised it.

The function is normally held in a separate workbook saved as an add-in but will exhibit the same behaviour if stored locally in a workbook module which is what I have done.

I have attached the file with explanations and instructions on how to reproduce the error which, on my machine, exhibits the problem.