I thought it was against the law to shoot/trap a cat - feral or not? I am not defending the cat, but, if its feral, it sees the ducks as a source of food, as does a fox. I am surprised at any cat tackling a fully grown bird, duck or hen.
I speak now as a feral cat owner, and a poultry owner too!! Last year, I lost young bantams, aged 6-8 weeks, maybe 6 or 8 in total. I never saw it actually kill a chicken, although, it did chase them while I was there, once or twice. but I am certain it was the tom cat!! This year, all the chicks wlll stay fenced in until they are old enough to cope. The cat has never touched anything full grown round the farm, and walks through the flock of hens.
I was upset, and annoyed at the loss of my chicks, but would never want to hurt the cat for doing what is natural to it.
A word with the CPL may be the best thing, and hopefully they can live strap, and take it elsewhere - hopefully miles from you.