This issue is now resolved.
Thank you very much to all those who assisted and offered advice.
The issue appeared to be two-fold; an authentication delay, not making the links available, which appeared to resolve itself with time, and then subsequently, a bug on the website where I received the 'internal error' page display. This bug now appears to be rectified and upon being able to complete the provided instructions, the process worked as described.
Thanks again for all the assistance.
I am now off to analyse my stats and target some gaps!