Dishwasher fills but will not trigger was...

After filling the dishwasher does not activate the wash cycle. I suspect that the level switch is not working correctly. The washer displays a warning light that the washer 'does not fill'. Opening the door shows the water level to the brim of the lower bowl. If I top up the water level before completion of the fill such that it covers the bottom box by say 5mm then it starts and washes ok but fails again after draining and re-filling for the rinse. Had a look for the level switch but nothing inside the washer except for some inverted 'U' shaped plastic tubes. Tipped the machine over to look underneath but no switch was obvious. The supply line filter is clean and tap on full. Fortunately it does not overfill!