Pork shank meat yield
Well, I am in position to answer my own question now. I used 6 shanks (I got two more as I got worried they wouldn't be enough) and fed 10 adults and two children very comfortably with some leftovers. 

My original shank weight was 6.8Kg and I had four front and two hind pork shanks. I would guesstimate they yielded about 300g x 12 people ~ 3.5Kg so, if ever anyone needs the info: 

Pork shanks yield about half their weight in pure meat

As an aside, I would advise anyone cooking pork shanks to separate them from their skins and salt and dry the skins separately in the fridge for a couple of days before crackling them on high. The crackled skins were almost as delicious as the shanks themselves (my recipe called for very slow cooked pork which turns the skin to rubber which is a waste).

Another aside, the bones from the shanks are INCREDIBLY gelatinous (and tasty). I kept them and made terrine that smells much nicer than using trotters.