I think the point is simply that if using the title (which is purely optional) then it is standard to use the first name. So Sir Stewart would be somebody else.

A, ok! Missed that nuance!

Full Nuance Here:

http://www.debretts.com/forms-address/titles/knight

Was that down-vote launched from the barricades you were manning? :)

Accuracy is independent of politics. Or, if you prefer to think of it this way, Know Your Enemy.