Are they the dreaded Red Spider Mites??

I'm almost certain they are (sorry no pics they just look too horrible for words nasty wee things..)
but I find them now and again lurking underneath a freshly laid egg. Not all of the eggs mind, only one and not every time.
All birds seemingly fit and healthy and looking good. So no outward signs.

The little blighters (spiders, not the chooks) only appear en masse in one part of the coop during the day and for the life of me I cannot find them anywhere else. Recently I covered the area so that they have no where to hide, but found them this afternoon loitering amongst the shavings.
I used some cheap and nasty stuff a few weeks back, but have now splashed out on Poultry Shield and gave the coop a liberal spray before bed time.

They are very annoying little critters and hope they bog off....

Poultry red mite  is different from the red spider mites you see on walls.  Red mite looks like tiny shiny red blisters or like tiny grey specks if they haven't fed, red spider mite look like tiny red spiders.
if you only have a few mites the hens wont be showing any signs of illness yet.  Poultry shield is good, diatom is good also, you can use it all of the time and it wont wet the shed through.
Use the poultry shield and diatom weekly till there are no more signs then diatom all the time to prevent re-infestation.

sounds like red mite 

check the hens for anything crawling, at base of feathers, around the vent, red mite don't live on the hens tho, just feed on them at night.

Wonder if they were just under 1 egg if it was the last one layed and the warmest?

Sometimes they are not red if they are not full of blood, then they are usually a grey colour.

These were black.

I can see I am going to have to take a picture.

that's gross Ruffle where were your marigolds? i'm still awaiting a red mite invation, i've got all the gear ready as i felt sure we'd get them sooner or later, i've heard the main carriers are wild birds, we have wild swans, Canada geese, coots and ducks land on our lake regularly, our ducks and geese go on the lake too and then come back to the yard to eat and sun/dust bathe/chill out, really don't know why we haven't had them yet, just lucky so far i guess and they can be the very devil to get rid of.