Thank you for your replies.

Ellie - do you not find by draining the pond each year to undertake the
dredging, that you end up with problems with algae growth, or is the
pond spring fed?

There are plenty of natural ponds / lakes in the countryside that seem
to find an equilibrium, and don't get dominated by sludge or algae - so
I wondered whether the sludge would eventually get to a level where it
decomposes as quickly as it accumulates.

Michael.