From tv.cream.org:
FERGUS THE FISH (1960-5)
ITV
FIVE MINUTE cartoon fillers featuring recidivist wooden fish swimming in a river with swishy cellophane plants in it. The plotline rarely changed - Fergus's mother (who was equipped with long lacy trailing bits) would leave, and give him a list of things not to do while she was out, to which he would reply "Yes mummy, No, mummy" in a burbly, must-be-a-fish voice. Then when she was gone, he'd do them all and get in trouble again.
Spooky! This programme sprung into my head at the weekend and i posted a question about it on televisionheaven.co.uk to see if anyone remembered it! The description from tv.cream.org is spot on! Thanks. you've stopped me thinking i was dreaming it all up!