I am the general manager of a car repair workshop, where all the employees are allowed to use our workshop for what are known as “hobby evenings” twice a month. Do we have to sign in and out of the staff register on these hobby evenings?

Yes, the staff register must also include unpaid staff. It can be a good idea to maintain an overview of these hobby evenings, or to mark them off in the staff register, in order to explain why the hours in the staff register should not trigger salary payments.