CivvyLurker I hear you loud and clear. I have had these tree bumblebees in my garden several times before. The ones in the extractor fan vent are not wasps, but they are not anything like those bumblebees in your photograph either. They have no hairs on them and they appear to be truncated in two halves similar to a wasp, very dark in colour with no visible pattern or stripes, and a shiny sort of sheen to their abdomen.They may be tree bumblebees which nest in holes several feet off the ground. I have them each year in different parts of the house or garage and have never had a problem with them, even when they were nesting under the fascia above the front door. The colony usually dies out by the end of July or into August so you shouldn't have to put up with them for long. If they come in through the open window they'll find their way out again if it's open wide enough, without bothering you. They're not interested in you, only the warm dry space behind your fan vent.
The little creatures are having a tough time of it due to pesticides and a reduction in food sources so if you can persuade your missus that they mean her no harm, they'd be grateful I'm sure.
You don't have to worry about the nest in the fan either as once they bees had died off it will dry out to become dust in the wind. I haven't had them nesting in the same place twice so you probably won't get them back next year.
Before you nuke them with fire, maybe contact the local beekeeper's association for a proper identification. Photo below, but yours might appear darker as their ginger fur rubs off as they get old.
Some interesting and useful info below on their lifestyle and cycle, plus how to live amicably with them:
Tree Bumblebee Bombus hypnorum - Bumblebee Conservation Trust
The Bumblebee Conservation Trust are collecting data on how common tree bumblebee nests are in tumble drier vents so you could let them know about yours as the extractor fan is pretty much the same thing as far as the bees are concerned.
Either way your plea for clemency on their behalf came too late, whatever they are, the deed has been done with the spray. I don't feel good about it, but it is a pain in the backside everytime you go in the bathroom not knowing what you will encounter and not being to open the window when having a shower, or a number 2.