Thanks for all your input in this delicate matter, after carefull consideration i have decided to refrain from telling neighbour.
I knew that by asking this poser on a wab 'BIRD' forum i would receive impartial and unbiased feed back

I feel that if i tell him that i had been watching a heron decimate his pond for the last week or so

it might well exacerbate the cool relationship we have.
thus , tiddles get's to have a chance of more fish suppers (unless said owner blames tiddles for the loss

) more to the point he'll probably keep restocking urgent food supply for the heron
ps; i just wonder how many replies this thread would have got if i had not included 'CAT' IN THE op.
poor innocent tiddles