There probably aren't many more of these around any more, but the
procedure may be similar for other models.

The fault was that over time it gradually loses suck. The filters are
clean, but it sucks less and gets hot. Today I went to pick up some
plaster while /she/ was out and got no response at all - so I took it apart.

This is what I found:
http://i92.photobucket.com/albums/l17/Number774/DC03%20cyclone%20cleaning/photo0037_zps64c13857.jpg

Since it had been a bit of a pain to do I took photos as I put it back
together. Disassembly is the reverse of assembly

This is what it looked like after cleaning:
http://i92.photobucket.com/albums/l17/Number774/DC03%20cyclone%20cleaning/photo0040_zps4c6ba74a.jpg

Put the conical inner cyclone back into the outer, and line up the
flange with the hole
http://i92.photobucket.com/albums/l17/Number774/DC03%20cyclone%20cleaning/photo0039_zps7063849d.jpg

Push hard to put the two bits back together. I found a bit of gentle
warmth (hot water) to soften the plastic slightly helped, but it still
took a lot of force - there's a ring around the inner cyclone
http://i92.photobucket.com/albums/l17/Number774/DC03%20cyclone%20cleaning/photo0038_zps1d376e32.jpg
that makes them stay together. A LOT of force - I put it on the floor
and pushed.

The rest of the grey bits go back together easily, but make sure you
have the screw holes lined up - the one in the housing
http://i92.photobucket.com/albums/l17/Number774/DC03%20cyclone%20cleaning/photo0041_zps347a9836.jpg,
the one in the pressure valve in the top
http://i92.photobucket.com/albums/l17/Number774/DC03%20cyclone%20cleaning/photo0042_zps24bef7dc.jpg
and the one in the cyclone
http://i92.photobucket.com/albums/l17/Number774/DC03%20cyclone%20cleaning/photo0043_zps40e9b01b.jpg.
Finally slide the entire cyclone back inside the housing, making sure
the bump lines up with the divot in the housing
http://i92.photobucket.com/albums/l17/Number774/DC03%20cyclone%20cleaning/photo0044_zpsa9fd54a1.jpg.

You can then replace the screw
http://i92.photobucket.com/albums/l17/Number774/DC03%20cyclone%20cleaning/photo0045_zps06d1f02b.jpg
and then push back the rubber seal that hides it
http://i92.photobucket.com/albums/l17/Number774/DC03%20cyclone%20cleaning/photo0046_zps1a495717.jpg.
The yellow clip just clicks back into place - the spring goes on the
plus-shaped bit.

When taking it apart the yellow clip needs to be prised out first, not
forgetting the spring underneath that will make a bid for freedom. You
need a thin bladed screwdriver to open the housing slightly so it comes
out. It doesn't look like it wants to, and you really need about 5
hands. This lets you get to the torx screw.

There's only one screw, but the grit and dirt which is why you started
means nothing will want to come apart.

Andy