Just bought a mini convertible and was told that the stain on the seat was due to the roof being left open.
I have done my research and Autoglym Interior Shampoo seemed popular. So bought some and with a soft sponge worked it into the interior.
Let it dry and looked at it a day later and stain appears to have gone. But weather and daylight hasn't been great so will check again to make sure. But happy with the results.