Hi Mark,
I reckon you have exactly the same problem that I describe at:
https://www.seahawk17.org.uk/owners-maintenance-rigging-boom.phpalthough yours is a slightly different gooseneck fitting to mine.
I used a club hammer to loosen my gooseneck. I'm not convinced the WD40 I suggested using actually did anything worthwhile. There must be a better chemical for loosening aluminium alloy corrosion. Hopefully, someone will be able to tell you what that is.