My car has started smelling again when I use the windscreen washer. It's done it before, and I finally got rid of it by spraying a bacterial agent into the vents in front of the windscreen wiper. However, I'm reasonably sure that it has to be caused by a blocked drain for the ventilation system, but can't find it - can anyone tell me where it is located?

 

Not sure I fully understand question. But blockage of drain sounds familiar.
In the older voyager model (about 1997) there was a drain of the compartment that holds wipers that used to block on a regular basis. To resolve, remove plastic covers of wipers("vents" as you name it) , remove leaves etc, use wire to unblock drain from above. For the newer model it is probably the same.

 

D'oh - obvious really - did this tonight - the drains (two of them) are about an inch in diameter. They weren't blocked, but I poured some cleaning agent down them in case there is anything nasty growing in the pipes.

However, the air intake for the ventilation system seems to be taken from the back of this area (under the windscreen itself). If any water is taken into the ventilation system, where does it end up, or are the quantities so small that they are just blown out of the vents? I wonder if this is where the smell comes from.

 

In my experience it ended up on de floormat on the passengers side.