Just come across this post as we have exactly the same problem. We've tried everything that has been suggested here and elsewhere, cleaned the filters, checked the hose, cleaned the cyclone head, none of this made any difference.
We've now discovered what the problem is, we're missing a seal from the side of the cyclone unit. When you remove the side where the cyclone unit is (the bit that you take off to empty the bin) there are two holes in it - the one at the top should have a seal in it and ours is missing. We borrowed a Dc03 from a friend, put their cyclone/bin unit on to ours (theirs has the seal) and the suction is completely back to normal.
A quick Google search revealed that this is the exhaust seal, these cost about £2 to buy, much cheaper than buying a whole new vacuum cleaner!