Yessss!
I did it.
Thanks for all the advice gents.
My mood has ameliorated with 10 points.
Will start cleaning everything, and then during the weekend I'll probably have time to reseal and test.
So the brass part was indeed pressed on. I had this idea whilst driving my bike back home from work through an Amsterdam park, that I could perhaps wedge a screwdriver under the brass part. I removed as much of the seal material in between the metal parts as I could, and then squeezed a small flat screwdriver head in, and started wriggling. It took about 10 minutes of wriggling, all around, and when it started to move, I used a slightly fatter screwdriver head, and then two screwdriver heads above each other.
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