Pete, I never encountered any mix of wood & plastic together, on any L1A1's I inspected or worked on in Service. Even doing inspections on War Reserve stocks, did not reveal any either!
I would only imagine, if in an 'up the sharp end' situation. if a lack of spares forced the change of particular component. Then it would be fitted to keep the weapon in Action. (if Safe!)
& when spares became sufficient, the weapon in question called in to the Armourer. For replacement of the correct component/ part.