Here’s the outcome in case anyone else has a similar experience. After finding out that the catch itself can’t be sourced from either Fiat or Hymer an after the garage had looked at it twice to see what they could do, I was about to take it in for the third time to see if it could be re-engineered. A pal of mine was present and asked if he could have a look. He looked at it closely, tugged at the ‘bits’ and then asked me to go into the cab and physically push the plastic bonnet release back in. Hey presto, the bonnet closed!
Thanks again to both of you.