When riding, a cyclist at a pedestrian crossing always has a duty to give way to both pedestrians and vehicles on the road. If, on the other hand, the cyclist gets off the bike and walks across the pedestrian crossing, the cyclist is now considered a pedestrian, which means that you as a driver have a duty to give way.
If you approach a pedestrian crossing which a cyclist is riding through, you still must lower your speed or stop if it is necessary to avoid a collision, even if it is technically the cyclist who should stop and give way.