I just can't fathom chickens out.

I really can't work it out. I always make sure that there is plenty of food and fresh clean drinking water for my hens, but they seem to prefer to drink from a muddy puddle! it must be the flavour.

I agree.  It's a case of prefering Eau Naturelle to Eau de Chlorine 

Once the tap water has stood for a while it will probably taste fine to them

Oh...  tell me about it    Same here 

my cat drinks from muddy puddles too, she always has lovely fresh water in nice clean bowl Why do we bother I ask myself!!

Horses are the same. Despite having fresh water in his stable the first thing he will do when turned out is drink from the trough. Althought the trough fills with tap water the additives evaporate as already mentioned. You could try putting water in a bucket, leaving it for 24 hours (you may need to e :)xperiment re time) and using that to fill their containers.