Who pays which costs in the event of minor repairs?

This is laid down in law. More information on this can be found on the central government website. The basic principle is that minor defects (irrespective of the cause) to the rental property that a capable tenant can repair themselves and involve low costs are to be repaired by the tenant at their own expense. Even if the tenant is unable to repair the minor defect themselves, they are obligated to call someone in at their own expense to repair it professionally.