Stripdown the cyclone. Remove the 3 screws at the cyclone top, remove it and remove the fins at the bottom as well as the shroud above it//. Now leave it in a boiling hot water bath with bleach or washing powder, any detergent you like and leave it sitting for a few hours. Now gie it a proper good rise and let it dry.
Does the filter still get dirty? If so, check the FDC seal The one at the bottom for the cyclone corresponding with the bin flap.
If the problem still persists then you surely need a new cyclone, the early DC07s have quite problematic cyclones, they either clog up with dust or get the filter dirty