Could it be that my Base Stations are too close together? they are pointing at each other on a diagonal 240 cm 2.4 m apart Each base station has a maximum recommended play area of approximately 6 meters by 6 meters (20 feet by 20 feet) when using two base stations. I won't be using my Hifi system whilst flying, so there won't be vibration whilst I'm in VR but it's a good point that I hadn't thought of. The vibration whilst blasting out Led Zep on my Hifi might move the base stations.

It says in the Valve guide that you need to plug the Headset into a power supply as well, but I only have 2 power supply adapters for 2 x Base Stations plus the Headset?? Do I need to buy a 3rd power supply adapter? There was a UK and US power adapter for the headset in the box, underneath. The Valve Index Base Stations require a power source to operate. They come with power adapters that need to be connected to a power outlet. If I can find something similar in the UK then that might be a good option for me, then I can have 1 base station mounted on the wall in front and to the right and one tripod behind me and to the left.I was giving to buy a valve index in August but after seeing so many people having problems, I decided to go for the oculus quest 2 which is wireless and trip free. It has done me good for nearly 3 months

Ensure that the base stations have an unobstructed line of sight with the VR headset and controllers.I would like 100% SS, no motion smoothing / ASW / frame doubling. 80 Hz / 80 fps. I have an i5 9600k @ 4.9 GHz and a RTX 2080 ti 11 Gb graphics card. My rig is in my signature below. Thanks for your lengthy and well articulated reply Fenris_Wolf. I will write of my experiences this evening before replying to your post...

So, I powered up my pc, connected to internet, opened Steam VR and allowed it to do all of the updates to the Headset, Base Stations and Controllers (firmware I believe). After a restart, the Base Stations booted up and I made sure there was clear line of sight between them, they synced and then Steam VR started the Room Setup, I chose 'Standing' and completed this process. In the Steam VR panel, every component was solid green - Headset, Controllers and Base Stations. The front right Base Station had a lit up A displayed on the front, I pressed the button and it changed to BI did a solo spawn on the Kuban map at Adler in the South East region of the map where the mountains are, I had 80 fps except when I was skimming the ground at treetop height where I was getting mid 70's fps. Like I said I run IL-2 graphics settings on Ultra, I took the shadows quality down a notch from Ultra to High and disabled the MSAA x 2. I tried the flight again and it stayed at 80 fps even at very low level over trees and towns.