Why do I need 40mm around the shower when fitting?

Once fully built, the showers will simply slide back tight to the wall or corner. However, as the pipework, electrics and connections are all located behind the unit, during the build, you will need to access to connect these

This means, you physically need to get down at least one side of the shower and have the shower away from the back wall enough to allow for this access at all times during the build. Once complete, slide the shower back against the wall and into its final position