They will be the larvae of gnats. Just leave them alone, they don't do any harm.

Thanks for this question/answer I wouldnt have known either if I had seem them in my butts.

I put a tiny amount of insecticide in the water (a teaspoon at most to 40 gallons) and that kills them off without doing any harm to my plants when I use the water. Do this mainly because we get mosquito larva in ours and they bite, well the adults do.
If you take the water out from the bottom of the butt, you can spray the water on top with WD40, or put a teaspoon of Cooking oil on the top. That stops the blighters from being able to breathe.

Like it Owd.b, knew that WD40 was brilliant but this is another great use for it! Thanks.

They sound more like rat tailed maggots to me....try google for pictures. Mosquito larva are less 'maggot' like.

Either way, stopping them breathing will kill them.

Perhaps I wont get them as my water butts are emptied almost as soon as they are filled?

You would not get them if there is a lid on the butt either.

Well thats good as I have lids on mine, thanks.

We only get life in the water butts with no lids and sadly no way of lidding them either. I drop a teaspoon of Bug Killer in whenever there are more insects than I care to see.