I've now ascertained that the problem is either temperature or time related. When the problem occurs it now appears the hyraulic pump fails to run (and I can't select gears). If I leave the car for an hour or so and the problem disappears ie after an hour when I open the door the pump starts and once its up to pressure I can select gears again. Could one fo the solenoids on the actuator be causing this problem? Does anyone know enough about the hydraulics and actuator sequencing to know whether one particular solenoid, say if it was getting stuck, could cause this problem?