406 3.0 V6 air con filter

This smell question has come up before in this group. From memory it is caused by bacteria and there is a spray (I believe from Halfords) that can be used to eradicate it. Someone else may be able to provide more info.

Yes, you get condensation forming on the evaporator, and this damp environment can induce bacteria growth.

My family irritate me by always opening most of the windows about 2 -3 cm when we get home. I think it is this sort of problem which is claimed to justify it.

Legionaires Disease is supposed to be fostered and spread through air-conditioning systems in buildings, originating in water-cooled recirulation systems (apparently nirvana for bugs). If a/c it can spread bugs it can certainly spread smells. And air circulation seems likely to take care of the smells if not the bugs.