Hi Steven.
I presume you know that the best way to remove the HPFP regulator is to first remove the starter motor, this will give you a lot more access, also do you need to change the regulator valve or is it the O-rings that need changing, if O-rings minimal cost £5.90 link below.
https://www.ebay.co.uk/itm/Rover-75-...19.m1438.l2649
Also a few pics below, if you notice in the photo's the large black O-ring is missing on the regulator taken off, it is replaced when adding the new O-rings, I have a feeling that this ring erodes as I did m find a few specs of the black O-rings on a couple I have removed.
The tips can be cleaned with carb cleaner before replacing back onto the main HPFP. good luck Arctic