I have also wondered this and have been asked many times, so today I made enquiry of Pilkington and here is the official Pilkington instructions
Thank you for your recent enquiry to Pilkington United Kingdom Limited
Pilkington Activ™ self-cleaning glass can be cleaned manually if it has been contaminated by non-organic materials. We recommend gently hosing down the glass to remove any loose dirt. If the dirt or dust remains, a mild, non-abrasive detergent (i.e. one that does not contain solids in suspension) and water solution is recommended. Abrasive cleaners must not be used. To wash the coating, apply the solution to the glass with a clean, soft cloth, sponge or pad and rinse thoroughly with clean water. Dry the glass by wiping with a soft, lint-free cloth. Take care to ensure that no abrasive particles are trapped between the glass and the drying device otherwise coating damage may occur. Steel wool or razor blades must not be used on the Pilkington Activ™ surface. Pilkington Activ™ then requires a short period of exposure to daylight to become fully effective, the self-cleaning property is not activated until the glass has been exposed to UV light from the sun from 4-7 days.
If this proves unsuccessful, one option would to be use a brush or cloth in conjunction with white spirits to remove a stubborn mark or stain. If using a brush, it is essential that the brush should comprise entirely of soft bristles. Furthermore, it is advisable for the affected surface to be completely wet to ensure that any hard particles are rinsed from the surface before, during and after the remedial action. Even for brushes with soft bristles, the use of a brush should be undertaken with great care and, in particular, avoiding any abrasive action. There are brushes available which have soft bristles around the outside but hard bristles in the centre and there have been reports that the use of such brushes have resulted in scratches.