Seriously Gordon he sounds like he's acting like a total a*se about this.
I know he's a mate, but he's also taking the mickey. Stand your ground and be very firm about it and if needs be put your share up for sale and just get out of it totally.
I have seen so many shares go down the pan and good friendships destroyed, that you really have to be careful about this if you want to protect your friendship.
Talk it over together seriously and state what everyone here has already put. i.e the basis for your "agreement" has changed through no fault of your own, so why should you suffer as he's moved away for a bit of nooky?
It simply isn't right that you should suffer in this case. So tell him to sort it, or you'll simply put your share up for sale.
Again, anyone reading this and thinking of buying a share, for the sake of your future sanity make sure you do have a good written agreement. No good in this example, but hopefully this will make others think twice. Especialy if you are uying an a/c with "friends."