Try to see those ones
open SOLIDWORKS as Administrator.
Make sure you're not saving to the default material database.
(C:\Program Files\SOLIDWORKS Corp\SOLIDWORKS\lang\english\sldmaterials\****.sldmat)
To make a custom material, you must create a custom library and copy a material/material library before editing.
I would recommend creating a custom material library in a shared location (network drive/vault).
Thank you for your help.
The problem has been resolved by moving the material database to a different location on the computer.
It appears that SW cannot edit the files in the default location. For some reason the Solidworks folder in the programData location has been ticked "Read only" which is affecting all the sub folder and files, and windows won't let me change it even though I have Administrative rights to the computer.
I believe this is by design. Since materials are used so heavily by any SOLIDWORKS user and so 'sensitive', SOLIDWORKS want to ensure everyone has the same 'default' library for all users.