Mark,
I have at last found Karel’s article which I previously mentioned plus another poossibly useful document, both in the Members only section of the website. Here are the links to save you having to search ……
https://traction-owners.co.uk/wp-content/uploads/2017/01/Tractionitis-Oleopneumatitis-_E_.pdf
https://www.dropbox.com/s/rv0o7sq4okgoyjk/15-6H%20Cars%20Hydropneumatic%20Suspension%20Technical%20Note%20600%20September%2054.pdf.zip?dl=0
Hope this helps,
B…….
Bernie