I've just looked in the P38 manual and can't see any mention of catalyst heat shields. I have a DSE so can't help by looking, so I can only give you some general/non-vehilcle specific info if that helps.
A catalyst typically runs at around 800 degC (and hotter !) and so radiates a lot of heat under the car (they can glow red hot). Typically, catalyst heat shields are fitted to the floor above the cat to avoid the cat radiating all that heat directly onto the underside of the car.
A heat shield is normally a thin lightweight piece of pressed steel that is held on the underside of the car directly above the cats by a few thin pressed steel mounting lugs. Typically it would be bolted to the floorpan with half a dozen M4/M5/M6 size bolts/screws.
Fitting should be easy if you have free access to the underneath of the car, can wiggle the shield between the cat and the floor and can still get to the shield mounting bolts.
Cost, it's a piece of pressed steel just a little bigger than the cat, it doesn't need any clever finishes or edges etc., cost should be £20/£30 each, however as it's a RR it may be a lot more!
Sorry I don't have any RR specific info, hope this helps .......
