I realise that the OP will have resolved their issue by now, but I just thought that I'd add to this thread for the benefit of anyone else who comes across it whilst researching the same fault (as I did).
My fuel filler flap sporadically opens unexpectedly; sometimes upon unlocking the car, and sometimes upon driving away. I also had times when it wouldn't open at all.
If the flap refuses to open at all, it can be opened manually:
Open the boot.
Open the storage compartment on the left.
Remove the first-aid kit.
This reveals an opening in the inner bodywork giving access to the rear of the rear light cluster.
Unscrew the large grey plastic bolt that retains the rear light (the light won't fall out). Don't remove the light, as it's quite fiddly to put it back in.
If you're quite flexible (and got a skinny arm), you can now reach right in through the opening and feel around until you get to the side of the flap actuator switch.
There's a small lever which you need to twist clockwise just a little bit and the flap will spring open.
I have tried it without removing the plastic bolt but I had great difficulty getting my arm out again!
If you can't get your arm in (and you can't persuade anyone with skinny arms to help), then go ahead and pull the rear light cluster out so that you can just reach straight into the cavity from behind (like James Herriot). It's just a bit tricky getting it back into place if your at filling station urgently needing to refuel!
My fault appears to be fixed now by replacing the actuator.
I googled the part number and found a Mercedes part dealer on eBay. It was about £30 and very simple to swap out. It was literally a case of unclipping the old one, unplugging the lead and just plugging the new one in. It took about a minute!
The new one had a different part number, so guess it's been improved in some way. The part you need is: 000 820 77 03