Well this morning I went to check my oil and top up the windscreen washers.
I noticed a plastic cap was off the small reservoir on the rear nearside of the radiator which I then realised was the engine coolant reservoir. This was completely empty. Rang Lexus Cheltenham who advised topping it up (took 1pint to half fill). Quick trip to Cheltenham car checked and no sign of leaking but reservoir now nearly full.
Strongly suspect that 4 weeks ago when car at dealers for replacement grill component the cap was not fitted back properly when the front of the car was removed.
The tank isn't pressurised so some water may have been lost, but apparently the cold level is normally only just above the minimum line and there may have been air in the system after the water pump was replaced last year.
As usual Cheltenham Lexus Service was excellent.