Clue: Leading pair in heretic group of mine almost converted Diana (2,7,3,1)
Answer: MY HUSBAND AND I
Explanation: Definition: Leading pair
This is the obscure one: Jan Hus was a Czech priest burned for heresy in 1415, so a "heretic group of mine" could be MY HUS BAND, followed by AND I, which is an anagram of DIAN[A]. It's a phrase associated with the Queen (hence "leading pair"), but in fact not used much, if at all, by her for many years; I suspect she may have been advised against it because it was widely lampooned