hi n and welcome if you place a womens old tights over the fill pipe it will keep a lot of muck and stuff out the but also if you can get to it safely try putting some fine mesh over the down pipe to start.use it often from the bottom as much as you can if you have a tap on the but.if not drain it and start again when it looks like rain.good luck.

Just keep using the water regularly to water your plants... Our rain butt has goodness knows what in the bottom of it, especially as himself soaks the large clay pots in it to remove the last of the soil. Our rain butt is uncovered and has never smelt bad in the 20+ years it has been in position.

You could try a mesh bag of horticultural charcoal, say 1 quart for every 10 gallons, changed quarterly.

kennyboy(above) has the right idea- I do exactly the same thing, but it's very easy to forget to change the tights over the bottom end of the fall pipe from time to time.