Further to my last message, unfortunately there are no actual dates mentioned in Harry's account. He mostly speaks about what happened in relation to where his unit was at the time.
This is the paragraph before the one that mentions the 91st.
'We had been in Armentiers about two months when I was out by the canal and saw an army issue cycle lying on the bank. Three days later when I was passing the same spot it was still there, so I thought if still there in the evening probably the owner had been wounded or killed, so I rode it back to the battery billet and put it in an old shed. It came in very handy when I made a pass out for myself* to go behind the lines. I had a surprise a few days later, my cousin had heard that my battery was in Armentiers..'
(* elsewhere Harry mentions how he used to forge passes for himself and mates!)
Harry was with the 100th Siege Battery RGA. From this it sounds like Cecil's death occurred about a month and week or so after the 100th had got to Armentieres.
There is no other mention of the 91st in Harry's account, but I hope the above answers your question.