The generic name of the product is Silicone Water Proofer, it usually comes in a 5 litre can and you brush it into DRY brick/mortar. It sinks into the surface but has a highly volatile solvent in it that evaporate leaving the silicone waterproofing behind. It does work - I have used it recently on the cement haunchings at the sides of a roofing valley. Where I can't reach the mortar goes green with moss but no sign of moss where I have treated.
Jewson's sell their own brand - about �25 per can.
Its only useful to treat bricks that get soaked by rain and hence damp that is coming through from the outside. It won't stop condensation IN your rooms caused by washing drying, showering etc.