Richard wrote
Is it me or are all non filtered ponds prone to get smelly without the
benefit of growth and sunlight? I'm hoping all will come good when the
UV starts filtering through. Otherwise, I'm looking at draining with a
bucket and replacing all the hard work of last year!
Any ideas?

Only if you let them get a layer of detritus on the bottom that will play
host to anaerobic bacteria hence the "rotten eggs" smell. If the pond smells
without you stirring up the muck then the water is bad/polluted and, yes,
you will have to drain, clean out, and start again.
Unusual in just one year though, is it under trees, does it get lots of muck
in it?

--
Regards
Bob H
17mls W. of London.UK