Removing Kawasaki TG24 head prior to fitting new?

I have a Kawasaki TG24 Brushcutter. I have purchased a new line trimmer head and wish to fit it. I cannot figure out how to remove the old - probably original - head system. I can fit the large spanner supplied over the collar from which the splined male piece projects to fit into the old trimmer head, but I cannot figure out what I am supposed to turn it against. Other systems I have worked on in the distant past would have had some arangement to insert an Allen (Hex) Key in to lock the shaft. I cannot see anything like that here.