I've had mine out a few times in the past couple of days. Takes about one minute to do. You're welcome to pop down to herne bay and I'll do it for you but it really isn't tricky. Shine a bright torch up to see the clip, use a small screwdriver to pull the clip down on one side. Then use a bit of plastic (or something that won't damage the vent or dash) to lever out that side of the vent a touch. Then, screwdriver on the other side for the other clip, and use another piece of plastic, or slide the other one across to pull that side out a touch. Then you can pull the vent out.... job done.
DSJ...thanks for the offer....unfortunately i'm in central....though...i might be down in kent in a couple of weeks.
if i havent yanked it out by then, i might take you up on your offer!
The petach method takes around 3 months and involves a lot of swearing.
On the other hand you could you a couple of dinner knifes like I did holding in a v shape to get both clips at the same time, then pull the middle vertical section with the other hand.