Renault Scenic ecu

Can someone tell me where the ECU is located in my Scenic and is it difficult to swop out. The Car has been idle for a few weeks and now will not start. It oopens as normal and redognises the Key, as soon as you press the start button the Radiator Fan switches on but there is no sound from the Starter, no engagement and no starting. When I press the button to stop the Car the Radiator Fan switches down to a lower setting but continues to operate. I have had the Car in a Garage today and their suggestion is that the ECU is gone - they had communicated with the Car and cleared down any faults that were showing up. They also had towed the Car and tried starting it in that manner but to no avail.