I believe that engineers are needed for a bus to go out in service:

Engineers need to sign defect cards on a morning when defects are identified,

Attending breakdowns and fixing faults,

Fuelling and shunting in the depot as well as operation of the wash,

PSV/MOT tests.

I wonder if a service reduction will be in operation during the strike days? I'm sure someone told me there needs to be a minimum amount of engineers for buses to go out in service however this may not be true but looking at the above list, things could go wrong quickly for GNE.