You can always try a tip I saw on Gardeners' World last year - poor a little oil (Veg, sunflower, Olive) on the surface of the water to produce a film.

The purpose of the tip was to prevent mosquitos and other larvae. But I'm pretty sure it would seal the pong in. But, they did suggest draining the butt first.

I suppose also it would create an airtight seal making it more difficult for algae to regrow.

Have a bit more of a research - at the end of the day, it might only cost you 50p to control the problem.

Spray the surface with WD40 is another tip, but only if you take the water from the bottom rather than the top.
Answer is simple, use the water up faster! Ours never get the chance to go bad, they are emptied too quickly.
Also I add a teaspoon of Provado to an oil drum size water butt, it kills the mosquito lava but is so dilute to does not affect anything else.

That's a crafty idea Owdboggy. Must try that. My butt water, out of 3 butts ,goes so quickly, like yours, doesn't get to smell. An empty butt is a good time for a clear out of any detritis in the butt as well.

My 200ltr plastic water butt (recycled chemical drum) never gets algae or skeeta lava in it. I sawed off the top of the drum turned the 'lid' over and placed it back onto the drum. This stops sunlight from hitting the water causing algae to form and stops skeetas laying their eggs in the water. To add weight to the lid so it would not blow off in high winds, I planted some small cotoneasters on top creating a small landscape (see my photos).
I draw water from the bottom via a tap. During summer (hot weather) I can empty the but in one week just watering my bonsai. During winter the full water butt is never touched or anything added and the water remains sweet.

Our rain water butt is open to the air has all sorts of 'stuff' land in it and never has a problem... Guess it depends on the ambient temperature!