I've only ever dredged my pond or when emptied bucketed the sludge around the flower beds.

I cant see how any treatment would remove the sludge. Some treatments make the fine particles clump together and float so you can skim it off but Ive never used them.

Is it a natural pond or artificial? If a natural pond the only thing you can do is to watch it become a nice little meadow in time. If artificial, drain it, then put a clothespin on your nose and boots on your feet and with bucket and shovel get the muck out being very careful not to damage the pond liner.

Thanks for your answers, I have managed to get some of the sludge out for now, but I know at some stage it will be a big job, keep well