That's odd. I have the exact same sander, and I found the extraction to be surprisingly good. On both sanding areas.
Mine goes across about 6m of 2.5" ducting, through a collection box, and then to a shop vac.
The only things I could possibly think of to add extra extraction would be the ambient dust around the area, so one of those fixed flexible pipes,
http://www.axminster.co.uk/stayput-extr ... se-ax22323, or a down draft table, or a hood somehow rigged up
http://www.axminster.co.uk/light-duty-v ... ood-950030, or a cooker style hood over the top
http://www.axminster.co.uk/big-mouth-dust-hood-200114, or a microclene filter next to the sander
http://www.toolpost.co.uk/images/MC760.jpg.
Whilst all these could help remove some ambient dust, I am not sure they will really add much to this specific tool, and could be some effort to set up.
I suppose the Microclene could be toted around the shop wherever you're working at the time.
Personally have an additional jet room air filter, and use the standard dust extraction on the clarke. Perhaps check you've got no blockages inside?