I have just read a post on facebook that Jeyes fluid can seriously affect dogs and can kill cats - even diluted.  Now on a garden forum it advises people to spray (diluted) jeyes around the garden to keep off cats.  Is there any truth in this?

Yes there is, especially so with cats. Jeyes contains phenols, cresols and terpineol compounds, which are highly toxic to cats and accumulate in the liver. It is one of the many disinfectants that must NOT be used around cats.

I am not sure how it affects dogs, as they are more efficient at expelling chemicals from the body.

Someone round here hung a cloth soaked in this stuff on a fence over 100' away and my eyes and throat were sore, the smell was overpowering for several days. Awful stuff.

A strategically employed super soaker (not the super duper kind ;-) )   is very effective at discouraging cats :-)     Just don't let them see you ;-)

> very effective at discouraging cats

I wish!  Young Eddie has just had his first cat chase.  We went out for his last visit to the garden and as I walked around and checked things a cat bolted from it's place where it must have stopped when we took it by surprise by going outside.  Don't know whether Eddie knew what it was - it was something that shouldn't be there and it moved fast - he has ignored cats up to now! 

> Someone round here hung a cloth soaked in this stuff on a fence over 100' away and my eyes and throat were sore, the smell was overpowering for several days. Awful stuff.

Seriously, not to be used anywhere around cats. They can ingest it through their paws, or lick it off their coats if they encounter it that way, even diluted it is toxic. It is really dangerous to them (as is tea tree oil, as that is also rich in phenols).

I use it once a year about this time to get rid of the green algae on the paths and concreted areas that's accumulated over winter, plus it disinfects in a way that my usual Mistral-type disinfectant doesn't, so it's a bit of a spring clean.
I have no cats and none visit the garden (this is not a cat-rich area by any means)  - but I still rinse thoroughly and allow to dry before the dogs go out afterwards.

I find that bio clothes washing liquid gets rid of all the algae in our yard and patio.   It also gets rid of smells :-)  even cat pee !

Still need to use mistral or bleach as a disinfectant though  ;-)