The message
from Pam Moore contains these words:

This is on a similar theme to "Stertilised water".
A friend has a rain-water butt which got very smelly.
She was told to put Milton in it. I told her I wouldn't use water
containing Milton on my garden!
Is there anything she can use which will remove the smell and not
render the water unusable. Had it been mine the water would not have
stayed there long enough to get smelly!
The butt is fixed on the wall with down pipe connected and not easily
accessible.

It shouldn't smell, if she excludes all light, mice and birds with a
closefitting lid, and a cheap drainpipe filter top to prevent
leaves/mice /birds falling down the spout from the gutter above.

I suspect a bird or mouse has got in somehow and is decomposing, The
answer is to drain, tip, rinse and refill.

Janet