Thank you cxfan1960 and again to everyone who has taken the time to help us.
I have just looked at flights from Beijing to HK with Cathay and there is one at 7.25 am with a 4hr 35 min connection or one at 8am (4 hours) or 9.30 am (2hr 35min) There is only one flight to Da Nang at 15.40. So thinking maybe the 8 am one and we can spend a bit of time sitting in the Cathay lounge, if there is one near to the Cathay Dragon terminal, had a look on the airport website but there seem to be quite a few lounges and its a little confusing as to where they all are.
This afternoon I have looked at every other flight combination and unless we want an overnight flight then these do seem to be the best option getting us to Da Nang at 16.40, which is a lot more reasonable than the VN flight.