This isn't quite a porter but if you're into them you should try an ale called Dark Island.
I've only seen it in Scotland (The Ensign Ewarts opposite the Hub at the top-end of the Royal Mile, Edinburgh) and at a beer-festival here (Halfway House, Brenchley, Kent) on tap, but I'm sure you'd be able to find it bottled somewhere.
http://www.legendslimited.com/darkile.html