Hi Urs,
I have found the problem and feel so silly because it is very simple. The installation folder in the Simulators section of the data provider was incomplete. It was: "D:\Program Files\Lockheed Martin". It was missing the final part - "\prepar3d v4". Once I added that everything is working perfectly.
I should have seen that earlier. Sorry to have wasted your time on this.
Thanks,
Graeme