Many thanks for the comments. I wasn’t able to get the arm fully off but was able to free it up sufficiently for it to work properly.
Lots of blaster spray, heat to the joint and tapping with a rubber mallet eventually worked. It has taken about a month to go from frozen solid to fully free! Happily, the tool looks good to go- all bearings are running free and I hope to be using it as soon as the new blades arrive.