All sorted. Lexus had a look and confirmed that I had no leaks on the radiator, hoses, expansion tank or the water pump. They also confirmed that they had changed the coolant in January 2015 when the car had a Major Essential service. I can only assume that the expansion tank was not topped up sufficiently. They have topped it up and it is showing at the maximum mark when the engine is up to normal operating temperature.
The issue as to whether the tank was filled with plain water does not arise for me as I have never had to top up since I bought the car in 2005. It has always had red coolant added when serviced.
Wissam I hope you get yours sorted.