Cold chisel to to get things moving would be the option seeing as all else has failed so far

It's off! Definitely looks like a combo of gasket sealant + old gasket. In the end it just needed hitting pretty hard with the mallet. I was being a little gentle as I thought there might be dowel pins and didn't want to jam the cover against them.

I said there were none. Didn't believe me?

And I'm aware there are dowels in heads, but they are rarely removed so didn't need mentioning