A few quick tips:
1) an airlock is never a good indicator whether fermentation is complete or not. If I bottled after my airlock stopped showing activity, I'd bottle some of my beers after 5 days. Not good.
2) More time, within reason, is never a bad thing. For a porter I'd let that yeast work for three weeks. Most of my porters get racked into a secondary for further conditioning (and lots of times with toasted coconut or cherry pie filling). Our yeast friends need a little time sometimes.
3) If your gravity reading indicates there needs to be a little extra 'umph' to hit the finish line, give your primary a good swirl and up the temp a little bit to get those yeasties going again.
4) Consider moving away from kits and developing your own recipes, even it you're doing extracts. I'd much, much, much rather use Safale 04 or 05 vs. Munton's yeast.