Hi pjoerg, many thanks for your reply.
I managed to locate, and found how to modify, the existing logos for existing airlines in MSFS2020 using GSX Pro.
Using a Graphics program that can handle DDs files, achieves the desired result.
I have changed the Ground Handling and Catering for 1 airline, and I use that.
What I have not been able to do yet, is create a new ficticious airline and add it to the list.
Very happy with results so far.
Martin.