Thanks for the reply's, The reasonn I asked the question is that I also have a1964 S type Jag and for reasons to long to go into I had to change the pump set up and the car is now using a larger Holborn Eaton pump from an early Range Rover..
Because I am a paraplegic I have to use hand controls. I think the (5503 pump) the uprated pump is making the steering too light and it makes the car hard to keep in a strait line as I have to use a steering ball with one hand and the other controls the brake and throttle.
Thanks for the link, it looks possible to lower the pressure by modifying the relief valve spring, I'm not sure how to go about it though. What upsets me a little is if I had thought about it I when I had the pump rebuilt I could have asked the builder to modify the pressure to around 800-8050 psi