What are you trying to open the plug with? Can you get a lend of a good fitting socket preferrably a six sided one and a tommy/breaker bar? That should shift it easily enough.
The plug is a 13mm bolt. I tried opening it with a 6 side socket (and rachet) - no joy.
Maybe it would be better with a ring spanner???
JUst bought soem Halfords "Shock and Unlock" so will give it a good spray with that and see if that is any good.
As I said - I have a feeling they put it in with an impact wrench - i.e. probably torqued it too much. I can't really get a breakers bar as I don't have the car off the ground. I can manage teh oil change without raising the car.
If I'm unsuccessful I guess a trip to a garage will be required. Just give them the oil and filter and ask will they do it