I ended up in a similar situation and did it by telephone ... obviously still needed to do lots of prep beforehand against the criteria but I felt the process was relatively quick. Submitting 'cold' in writing only would have taken me much more time I think, finessing the wording etc, and I would not have had such a good sense of whether I was meeting the criteria or not.

Hi and firstly good luck on your planned upgrading. I recently had the telephone assessment. In terms of the preparation I wrote out examples of where the HR contribution had made a strategic difference and the CIPD did give pointers as to what the interviewer would be looking for. I preferred the telephone as it felt more personal and allowed me to expand immediately on my experience if the assessor needed clarity which I feel is the main advantage over written submission Good luck whichever route you plan