My understanding of PAS2035 is that if you carry out work that changes the air permeability of the building you must ensure that there is a compliant ventilation system in place. If a PVCu, single glazed window, with a draught seal, is replaced by a PVCu double glazed window I don't believe you are changing the air permeability and so wouldn't need to install a ventilation system? However, if the new windows had trickles, and the old ones didn't, you are changing the air permeability in the "right direction". Would this also commit you to installing the rest of a ventilation package?