Discovery td5, 2002 pre facelift 5 seat GS.

Just had it serviced and garage has advised that the front springs are corroded, just the springs and when I quizzed them they confirmed that there was no sign of breaks or fractures and the areas where they are located and housed are sound and free from corrosion so my question is at what point does corrosion of the spring itself become a problem.
Thanks in advance for your advice.

The corrosion is a problem when they snap which will be many years from now most likely.

Have a look round, most coil springs which are exposed will have surface rust.

handful of grease wiped over what coils you can see will stop it dead, or if want to stay clean aerosol motorcycle chain spray grease.
As a rule land rover coil springs dont break as they are same size fat coils top to bottom unlike lots of cars that have very thin coils at the ends which is where they break.