I've fixed it!!!!!!!
You'll never believe what it was.
The first problem I had, with the error messages at the start of the thread was caused because not all the files were removed when I uninstalled terragen preview 3 for some reason. It left some files in the virtualstore and a couple of other places.
Once I'd totally removed all the files and installed preview 4 the new problem I had was that the software just wouldn't run, no message or anything, this is what had me stumped......until now.
The problem was that I hadn't copied back over my licence key to the Terragen folder after removing everything when I had the first set of problems. I was reinstalling so many times I just thought to myself I won't keep copying over the licence key until I can get the software to open. I assumed, and this is where my problem lay, that if I tried to run Terragen without a key I would at least get some message saying that the software requires a valid key or something. If I had received some kind of feedback stating that the key was missing the problem would have been solved in 5 mins, so maybe this could get added for numpties like me
Just thought i'd share in case anyone else comes across this.