There is an allen key under the flickmixer handle. Undo that and the handle will lift off. Remove the chrome cap, and under that will be a brass nut. You will need a pipe wrench or 12" spanner. Gently undo this and the cassette will be in the chamber of the tap. Lift it out and buy a new one. Reverse all of the above to put it back together. If it does not have a cassette i.e. Dorf 1 or 2 flickmixer, there are replacement washer and seal kits available.
Then write to Dorf and tell them to but a damned instruction sheet on their web site. I had exactly the same problem trying to fix mine i.e. no info is available from Dorf themselves.
Cheers
Tony Eldridge
June 2008