The Lockheed McDonnell 3-12 is a fictional aircraft model, and G-ERTI is said to be this type of aircraft.
In the real world, Lockheed Martin and McDonnell Douglas were two real aircraft manufacturers.
The plane used most often to represent G-ERTI (a Lockheed McDonnel 3-12) is actually a McDonnell Douglas MD-80, despite this plane being bigger than G-ERTI's apparent 16 seater capacity, which is more likely to be a slightly modified McDonnell 220 .