DCO4 Dyson keeps cutting out. ?

We have washed the sponge filter above the waste collection tube and it still does the same.
I notice what appears to be a filter below the waste collection tube and above what must be the motor (contained in a circular strutted structure). Is this likely to be the problem? If so how do I get into it?
I am now also aware that you can replace the thermal cut out device on these hoovers.
Suggestions please.

Alvan White
August 2009