Gaz, when you undo the screws a rubber plug should come off with each one. You then place the scoop on the top of the bonnet grill, then screw the grommets in from the underside of the grill.
Do you not have the plugs/ retainers?
No rubber plugs
The screws look like ones you get with flat pack furniture
Think washers should be able to hold them in place
Gaz, I have a Uno bonnet scoop that fits in a similar way to the one on the Panda. The retainers are simply small blocks of hard'ish rubber, similar to the rubber you get on a rubbing block. I'm sure you could find something like this then you could cut some chunks off it and shape them to suit, make a hole through the middle of each one and that would do the trick perfectly. Use washers between the screws and rubber blocks, but don't just use screws and washers only with the scoop else you'll scratch the paint/ damage the metal.