I have just changed all 4 glowplugs on my Megane 1.9dCi and although its a bit of a fiddly job, its not too difficult.
I'm not too sure about the access on a Scenic but on the Megane, you need to remove the large plastic cover which sits on the top of the engine - just remove the three bolts which sit underneath small plastic covers. Once the cover is off you can access the glowplugs directly.
See attached photo, where I have indicated one of the white glowplug leads with a yellow arrow.
As I said it is a bit fiddly but one thing I should add is that I did end up buying an addition to my toolbox - a set of glow plug sockets. (£20)
It's just a case of removing the glowplug leads - on the Megane they pull off, some are connected using a nut/washer arrangement.
Then (as I said using the glow plug socket set) remove each one in turn and replace with a new one.
Let me know if this is of any use to you and I'll try and help some more.