I think you are misreading Pc West's handwriting. In his log, he wrote "checked by GPO", not "called by GPO", and so the word "by" isn't a later insertion.
However, the line "0413 informant @ scene." does look as though it were added later, as it is unnaturally close to the lines before and after it.
Jeremy wasn't at all sure how long his call lasted, but guessed its duration was about five minutes. There is nothing in the logs to suggest it took 10 or 11 minutes.