Hozelock do a water butt container kit for this purpose for around £20, but there are also many other irrigation systems around as well that will do the job. have a look on Amazon or ebay or rig one up yourself. I don't think that too much pressure would be required for a few drips of water and so I would think that the pressure of the water in the butt would be more than enough.

Interesting .. added to GoYpedia Water Butts.

Thanks. I've located the water butt container watering kit on the GoY garden centre. Just what I meant!

Great ! :o)