Rob, I have had knee pain since my teens. I was taught a downhill technique b6 an old friend (sadly now no longer with us). He had severe knee problems, but continued walking well into his 80s. His doctor said that it was physically impossible for him to walk in the hills like he did, but he still managed it. To take pressure off the knees you walk a bit like a race walker with bent knees and loose hips.lean forward and let gravity help you down the hill. This technique looks a bit strange and takes a bit of getting used to, but is effective once learnt. It is also a fast way of descending and is good for unstable ground. The way most people descend involves resisting gravity, leaning back and putting high forces through knees and ankles. This way reduces the forces and hence the pain. I am sure someone else here will have a name for this and can point you to a YouTube video.