My first ds started to crawl 3 days before he started to sit.
Ds2 didn't sit until he was 9 months and crawled at 10 months
Ds3 started sitting at 9 months and crawled at 11 months

All babies develop at their own pace, ds3 isn't fully walking yet at 16 months, but i'm not worried yet, he's getting there slowly, but if i took him to a MCHN they would have him booked in to see a peadatrician because of it.
Use your mothers instint, it's what i go on and it's never let me down, no-one knows your baby better than you.
I stopped taking my kids to MCHN because they were rude and pushy, i take my kids to our gp for all of their checkups.
Good luck