Dear [Boss's Name],

Athough I finalised the initial draft of the report in time, some additional points of investigation came up. I didn't foresee this, but I should have kept you in the loop as soon as I knew of the delay, for which I can only apologise. 

Given this unexpected avenue of research, I hope it's ok with you if I get you the final copy of the report by the end of the week? I just want to make sure it's as thorough and complete as possible, and the extra few days would allow me to ensure as such.

Best wishes,
[Your Name]
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