Rather than blindly ripping door card off, I tried what both you and "Rupert" could have done - a quick internet search.
Pretty quickly I found it is a common problem, with lots of sites and forums mentioning it.
I THINK you may have the kiddie lock turned on, on that door.
Read the first few posts here:
.
https://www.renaultforums.co.uk/threads/2002-megane-rear-doors-wont-open.100396/
And a few posts here
https://www.meganeownersclub.co.uk/forum/viewtopic.php?t=21856
More, from YouTube:
https://m.youtube.com/watch?v=k55qRna7918
https://m.youtube.com/watch?v=BOhtiCnfOS0
Loads more out there for those prepared to look
Fghhdds, January 2020