I thought I would share what I have done with the water butt I recently purchased.
http://www.cpukforum.com/forum/index.php?showtopic=55843&hl=
1: Rainwater input from downpipe.
2: Air from aquarium pump (located inside flat).
3: Overflow from filter returning to water butt.
4: Overflow from water butt to waste water drain.
5: Gravity flow from filter to leant to plastic covered greenhouse type thing.
The small black bucket (painted to slow algae growth) seen atop the water butt has a
plastic bowl, with small holes drilled in it, inserted in the top and trimmed level with the top edge. Through the bowl, the base of the bucket and the lid of the water butt is fitted a rigid plastic pipe extending 2/3 the depth of the butt (approx 1 metre).
The air feed tube from the pump decends down this rigid pipe stopping approx 50mm from the end of the pipe. The rising air inside this rigid pipe draws water up and into the bowl of which is then filtered by draining through a filter medium. I am using scouring pads as my main concern at the moment is grit and lichens, although with a little adapting I see no reason why sand or activated charcoal or any filter medium that addresses your specific woes could be used.
Once the filtered rainwater reaches the level of the outflow pipe in the resevoir under the filter bowl it is gravity fed to the lean-to house.
Below is a picture of the pump (I used 1 outlet of a dual outlet 3watt adjustable pressure aquarium pump) £12.99
Rainwater kit under £20.00
Sundries(tubing etc) £11.00
please excuse the mess, the pump sits in the corner of the room with my shredder and whatever banal, vacuous comunications these multinationals insist on repeat sending me despite requests to be removed from mailing lists (Rant Over, Promise)
I worked out that using a digital plug in timer a 3w device running for 3 mins in the morning and 2 mins in the evening (ie 5 mins a day 365 days a year) will cost approx 1.2 pence per annum (I know, I had to sit down after I saw that) 1.2p per annum leaves plenty of CP buying power !!
If anyone wants any further details or I have not made things clear, please do not hesitate to ask.