Thank you all for your replies. I have managed to squeeze another couple of days out of my itinerary and can now do 3 nights in both Hong Kong and Beijing, which is what I originally wanted.
The reason I want to do 3 nights in Hong Kong is to do a day trip to Macau and a couple of other things.
Really looking forward to Beijing too!!
As regards Ho Chi Minh v Hanoi, I have already booked my flights and from people I have spoken too Ho Chi Minh sounds like it is worth a visit too.