Question Author
Wow - what a great number of answers. Thanks - and yes, dtgblue - I have had a very merry Xmas. Hope you all did as well.
I was going on the assumption that the only people who replied would be those who knew about the Magpie (an online crossword publication in the manner of the times Listener) and therefore knew the crossword to which I was referring in the title. Since this assumption was wrong (but not unjustified - the only people who contribute to Listener threads are listener solvers) I should have been more specific.
Never mind though, for I have now solved it anyway, after trawling through a wiki list of tennis players. Shaneystar - the letters in this case belonged to a jumbled clue, so I had originally no idea of their order. Gen2 - you have it correct. An Unch is an unchecked letter in the grid. Are you a listener solver? Yes, dtgblue, this is (or perhaps now we should say was) a strange thread, made even stranger by the first reply. Who mentioned churches?!
Anyway, thanks for the replies, even if they were confused.
Can't leave without answering the original question, I suppose.
The Unchecked letter in question was an L. (I know that won't help any of you, but at least we have closure). Happy new year, everyone!