hi andy, the G29 and the M420 both do almost exactly the same thing, in most firmware the commands are interchangeable.
when you run a G28 to home it disables mesh bed leveling(I know there are firmware options to change this but Default behaviour) You must run either a G29 or an M420 for Mesh bed leveling to be enabled.
You can enable it by running G29 which will probe the bed, or G29 S1 which will read saved mesh from memory slot 1.
M420 is similar but each with there own mesh options.