Lets get back to trying to help 
I am not familiar with this vehicle but, it is quite a few years old now and, with my older vehicles I have sometimes had trouble with the feed/drawback pipes from the expansion tank to the rad filler neck. They sometimes perish and it is prudent to check their mechanical integrity. Even a small pinprick hole in the rubber drawback tube which is fitted under the expansion tank cap will mean that coolant cannot be drawn back into the main system. Also, as previously advised, a new rad cap and inspection of the radiator filler neck is essential. You mention that a new rad has been fitted but, even a small mark or distortion of this brass rolled profile will stop the rad cap from sealing effectively
I am not familiar with this vehicle but, it is quite a few years old now and, with my older vehicles I have sometimes had trouble with the feed/drawback pipes from the expansion tank to the rad filler neck. They sometimes perish and it is prudent to check their mechanical integrity. Even a small pinprick hole in the rubber drawback tube which is fitted under the expansion tank cap will mean that coolant cannot be drawn back into the main system. Also, as previously advised, a new rad cap and inspection of the radiator filler neck is essential. You mention that a new rad has been fitted but, even a small mark or distortion of this brass rolled profile will stop the rad cap from sealing effectively
