This is a very interesting and informative link, LadyA. esp.in combination with the enlarged photo of JaffaCake's mites. Especially this sentence;LadyA wrote: ↑Mon Jun 18, 2018 6:54 pmI think your most likely culprit may be these: Harvest mites. Absolute bug***s! When I had outdoor cats, they used to sometimes get them. They are very tiny, almost invisible to the naked eye. But, they do congregate on grass, shrubs etc. https://www.the-piedpiper.co.uk/th5i.htm#harvestmite
I'm scratching like mad, reading this thread!!
'Heavy infestations, however, are often found to be sharply localised; a heavy infestation may occur in the gardens of one or two houses in a village which is otherwise free from the pest.'
Our dog is prone to getting harvest mites, but as the link says, usually later in the summer than June. The diagram in the link above seems to suggest a fatter, rounded body sort of mite, compared with your photo, though.
If it's possible that it's another mite that feeds on plant sources, does the evidence point to the large eucalyptus tree? possibly blown on the wind to window frames, or carried in on clothing etc by humans? Is the tree itself heavily infested?
At least, if you can get someone to come and have a look at them, you'll have a photo, a lot of research and some theories to share with them.