Thank you very much for that. The last link I have used for a couple of years.
The first link is very informative, and detailed. It supports my point (and therefore evidence for my friend) - but it doesn't explicitly state 15 days up to the due date; yes, I fully accept that it does say "Submit a request in advance before the due date for the next report within 15 days" - and that is good enough for me.
However, I am dealing with friends where a beer often rests on being able to prove something right or wrong. I just know those friends will say.... "Ah yes, it says within 15 days but it doesn't say you can do it in the last 7 days"....I know, change my friends. Ironically, that old calendar does show a tick for all 15 days.
Thanks again, I really like that first link.