Heat gun, hairdryer or an oven. All work, but care should be taken to not overcook (melt) the plastic when using a heat gun or the oven.
If using the oven approach, place the headlight on a piece of wood, not on the metal shelf. leave the door partially open and don't leave it unattended. And dont burn yourself :-/
Used a Hair-dryer. Worked a treat. Thanks Fellas. Still managed to burn myself lol. Any way to remove the goo stuff? I got a few stringy bits when prying it open land on the plastic inserts inside the headlight. No biggy as its only a little bit but it won't come off with just trying to peel it off. Don't want to scratch the plastic. Any chemical I could use? James.