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.
You can take a look at one of those actions (32 -38), add an additional character function for the Company table.
- Adam