I have had mine replaced by Jaguar, albeit after many many months of constantly pushing them for action.
Details of the content of SSM74031 as follows:-
Issue: A customer may complain of corrosion appearing on the Under-body braces.
Cause: Insufficient Zinc-Nickel plating on current components.
New spec parts include a Zinc-Nickel plated layer between paint protection and the metal substrate which provides a greater level of corrosion protection for exposed Under Body steel components, including protection against stone chipping and other environmental impacts due to Zinc-Nickel hardness properties.
Action: If any of the Rear Under body braces are exhibiting red rust corrosion, replace them with the new part number as listed below, and submit an EPQR with photo’s attached of the concern and quoting this SSM.
It is interesting when it talks about "Insufficient Zinc-Nickel plating" given the rust on mine it would appear that the ZN plating on my original parts was none existent.