Rodsstuff,
Just to check that I'm understanding you correctly, you are in the Edit Lead screen, Activities section and clicking New Activity?
If so, that will bring you to a pop up of the New Activity screen. The actions on the save button for that screen do not have the character function "Replace <<merge>> fields" for the Company table. You will need to copy and edit the CRM_NewActivity screen to add this action. This is why I'm assuming your other merge fields show correctly. You can see from the screen that the Company table is not listed.
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You can take a look at one of those actions (32 -38), add an additional character function for the Company table.
- Adam