What if the harasser is a union officer or rep?

Just like any other employer, unions are legally liable for sexual harassment (and for all other forms of harassment or bullying) by employees in the course of their employment. They are also likely to be held liable for harassment by workplace reps where that harassment takes place when the rep is representing the union (Unite the Union v Nailard [2016] UKEAT/0300/15/BA). The union should have a policy of zero-tolerance of all forms of sexual harassment and procedures in place and the matter should normally be referred to a senior national officer.