Have changed the throttle body with a genuine fiat one and the fault still comes back what else could it be? I have checked the wiring as much as I can. Has anyone ever found something else at fault.

Likely is a wiring issue, did the new throttle body come with a shirt link cable to the original loom? The connectors in the original housing can become very tarnished and won't maintain a reliable connection. If there is a link loom with tge new body suggest cutting off the in line connection blocks and connect the wires directly by some method.
On ours, there were also tarnished connections in a big black connection box under a headlight on the passenger side, worth opening them up and inspecting all the pins, as I traced the throttle wiring through there when fault finding the same issue.
A new throttle actuator and making good connections fixed it.

What part did you use exactly?
I used the Pierburg one and while the connector looks the same, the pin configuration is different. I had to buy an adaptor cable (cutting the original connector is also an option but I wanted to do it right). Also the connector's contacts are really sensitive, I had a bad connection due to putting a multimeter probe into the contacts, luckily it was easily fixed by bending them a bit back.