Please note I am by far not a professional but have had many a sessions with a few and did my share of research and learnings. With that disclaimer out of the way,...
Always give the leather a cleaning with a dedicated cleaner. Try to steer clear of an all in one.
- Give the leather a hoover, especially in the crevices with a soft brush attachment and ensure all debris is removed
- Use a dedicated cleaner and soft boars hair brush to help lift the dirt. Use a clean cloth to wipe up and do a small section at a time. Be sure not to put too much pressure.
- Now the leather is ready for any leather feed/conditioner but use sparingly.
I've been using Swissvax on the Morgan and other cars. I did a mates BMW with a deep red interior and it literally went from looking OK to brand new. They also have something called Elephant Fat that isn't something you want to use too often. Like leather conditioner, it's not something you want to use weekly else your leather starts to look wet and greasy. It should look matte if done properly.
Many great Swissvax vids on youtube. You don't need to use their product but their methodology is a good one to follow, IMHO.