Tricky, defamation? Yes. Your comment makes it seem like his uncle wasn't on board the Antelope and that he lied in the Chamber. PMQs is a stressful (I nearly said taxing) time for any PM. If it then also gets emotional because it's close to home, a faux pas is always on the cards.

All over asocial media folk are posting to the effect that it was a down right lie and his Uncle wasn't aboard. Exactly the meaning of your post. THAT is defamation. So, in answer to your "defamation" question, absolutely YES!!