Just got in after fixing the Parking aid
Storme, those photos and advice on the location were exactly what was needed. As I explained my sensors would all 'click' when the car was put in reverse but there was no sound.
Having eventually found the buzzer, thanks to Storme, I connected a small speaker in place of the original, put the car in reverse and .....Beep
The 'buzzer' is Fiat Part No. 46 555 998 and when I checked the price on-line it was a whopping £24.95
The buzzer is just a Mylar & Paper Speaker, 8 Ohms 0.5W
If you can solder, which I can, then you can get one from Maplins for £1.89 see this link
Storme thank you so much for the offer of a buzzer, but I am now sorted.
If you have a multimeter you could test the buzzer output from the control box. Disconnect the buzzer, set the multimeter to the Volts dc range and put the car in reverse with the ignition on and it should then read ~12Volts with you stood near the rear bumper. This will confirm the control box is sending out the correct signal.
To check the buzzer, set the multimeter to the low Ohms range and check the resistance across the connectors. It should read ~8 Ohms.
The control box pictured in my first post is in the boot behind the large plastic trim at the rear of the boot, the box is on the passenger side.