Just as a way of balancing things out or getting an idea of others - my youngest - Devon only sat unaided from 8 mths , he is now nearly 9 mths old and still has to have all the sofa cushions all round him in a circle as he will topple or flop back onto his head! He can roll over onto tummy (but not back again- needs rescuing!) but chooses mostly not too, lays or sits only if I pop him into sitting position. He has just started trying to correct himself a bit , so that's good. He is not crawling or comando shuffling at all, he hates being on tummy and tends to lay his head down. I'm not worried as for my kids he is right on target, they all waited till 8 mths to sit and 10 mths to crawl, then my others walked at 12mths, 13 mths and 16 mths - he will do it in his own time.
Do run it by your HV but try not to worry, each child is different and has a different set up around them, numbers of kids, their own want to do things , etc For example my first boy could only say 3 words at the age of 2 years 9 mths , and was on referal from HV for it, but his fine motor skills and hand eye coordination was apparently outstanding right from his 9 mth check, so yayy for him.