Take it apart (sponge out of plastic) and rinse both under cold water. Squeeze water out of sponge until water runs clear. Then leave both parts to dry for at least 12 hours before putting back into machine. Supposed to be done every 6 months.
I think I mean the hatch, Lowryn - the filter is inside a plastic drum thing? In my manual it looks as if you take the drum out but maybe you open the drum and the filter is inside a plastic housing? (I need someone to come here and do it for me!)
I couldn't find the manual online either! Only a copy of the actual paper one to buy at £2.50...not doing very well today...somebody do me a link please? (Or tell me what heading I want on the Dyson page.)
Jan look \link[http://www.dyson.co.uk/aftersales/help.asp?article_id=32&hist=,,1\here} at the dyson page and click on the DC04 user thingy.
If you don't have this model look back at the customer services bit and pick your model.