That is correct, when you're making the appointment it only shows you dates from 4 weeks before your limit of stay is up so you can't apply in person any earlier.
Update on my situation: appointment to collect was tomorrow but was walking past Immigration Tower this morning so gave it a crack. Waited for around an hour and left with my new visa. Appointment 21 Aug, got visa 6 Sep. Happy days.
One tip to share is that as I mentioned previously I was asked to resubmit my letter from employer with it specifically addressed to The HK Immigration dept, which I did. That letter went missing though and wasn't in my file this morning, but thankfully I'd gotten them to stamp a copy of the letter as proof of receipt. So if you submit any other docs either have a duplicate original or take a copy that they will stamp to confirm receipt.
And thanks again @asianexpat0