However, search as much as I could, I could just never find anything to match the ridge lines in the background.
It wasn't until just the other day that a particular ridge line struck a deeply buried memory in my brain and I eventually managed to finally place the filming location in Pak Shek Au - a small hilly area located between the Lok Ma Chau border area and Castle Peak Road, just down the road from Ho Sheung Heung.
Ho Sheung Heung and its neighbouring hill, Tai Shek Mo, you may remember as being the location of the filming of the fight scene between Sammo Hung and Bruce Lee at the beginning of Enter the Dragon, and it turns out that Tai Shek Mo was also instrumental in me being able to identify this location to. That's it at the back right, seen from the west, in the first picture below.
Now, these days this whole area has greened a lot but has also suffered from the sort of unchecked illegal development that blights much of the New Territories - basically a mish-mash of concreted parking areas, container storage, light industrial workshops, plant hire and other various blots on the landscape. This is a big shame when you see how empty and open it was back in the 1970's.
Now, we already know from some of the great spotting Andi has done that nearby Ho Sheung Heung has been featured in multiple films, including many Shaws productions, and seeing that this place is very close then it's easy to imagine it being a popular spot for filming here. I need to check but I believe there are many Shaws films that feature these same hills and dirt tracks (I'm pretty sure Come Drink With Me is one of these considering the last but one picture Andi has put in this post that shows Tai Shek Mo and its military insignia).
Anyway, mystery solved. Here are some more grabs. The first one shows a hill in the background that I've actually been to the top of without realising it's the same place, it's Ki Lun Shan. Somewhere hidden between there and the actors is the 1970's version of the Castle Peak Road (a much smaller version of the current one, but still a main thoroughfare at the time).
Ki Lun Shan in the background