Finally got round to fixing this. Bought the new part and did it myself as I wanted to add glue onto the new one and I doubt a dealer would do that! I used a heat proof plastic glue and put it on all the clips and also the underside parts which are in contact with the plastic on the dash. I'm hoping that will be the end of it as if it pops up on the drivers side again theres no way I'm going to be able to pull it out now its glued!

If your dash needs to be removed (which it might well need to be due to a variety of reasons) then this trim needs to be removed to get to 3 mounting screws.

Ah well.. It's done now and looking around the forum I'm not the only one who has glued this vent down. I expect you could prise it off if you removed the front windscreen, although I'd expect I'd then need to buy another new part to replace it with. Not going to worry about it anyway I didn't really see any other way to do it as its design fault and unless fiat redesign the part it would just pop up again without the glue.

there is a way I do it which just involves a slight mod however as you say it's done. Worse comes to worse as you say you can break it to remove it, no need to remove screen though.