Well, not more than half an inch!

Looked around on here & found the solution - sort of!

The culprit...


A plastic casing which holds one end of the cable & acts as a guide, allowing the cable to be pushed.

With the fuel flap closed, the cable is pushed. The other end of the cable has a mechanism which, when the cable is pushed, has a reversing lever which pushes the 'bolt' into the mechanism.

With the fuel flap open, the system is 'relaxed' and the bolt is out - not allowing the door to be opened.
A spring on either end of the cable keeps the whole system under constant tension whilst the fuel flap is shut - is it any wonder a cheapo bit of plastic couldn't cope?
TBH. I'm amazed it lasted this long!

So, spent half an hour 'fixing' it - at least the door now opens - just got to make sure the fuel flap is shut!

Seemingly, it's a precaution to make sure you don't open this door whilst the fuel flap is open.
I can only assume the 'safety' aspect is to prevent someone breaking a fuel pump (?) since the sliding door would simply close the plastic fuel flap ?

I figured, since I had to do the job, I might as well take pics for a guide...
https://www.fiatforum.com/doblo-guides/337889-nsr-sliding-door-refuses-open.html?p=3361478