From what you have said Gerald, I am placing my money on the filter being blocked. When my Rover's filter blocked, it dumped fluid everywhere. Changed the filter and all good again.
I think you will be very lucky to find one from Repco, but no harm in trying. I bought a genuine item years ago that came in a Land Rover box, dear as poison.
Having said that, I see that Mark Gray has them for sale, very reasonable price too.
FILTER POWER STEERING RESERVOIR - Rover P6 Parts supplied by Mark & Angie Gray (roverp6cars.com)
Ron