My strong personal preference in this case is Atom, although there is of course a case for either option being correct. However, if K thinks the latter, who am I to suggest otherwise?
It HAS to be atom IMHO. 'Our letters' means the letters we as a nation use. Def of atom = very small quantity.
Reasons it can't be item: there is no anagram indicator (half of the letters of The Times = THET or IMES). And, of course, 'very small quantity' is NOT a definition of item, by any stretch of the imagination.
Thank you everyone for your considered opinion. It would never have occurred to me that it was anything other than 'atom' if kayakamina (whose opinion I greatly respect) hadn't suggested item...and then there was that pesky word 'our' - although that (me, my, I , us etc) is more of a Guardian device than Times.
On balance I'm going to stick with atom especially after quizmonkey's rationale.
Very grateful for all your thoughts.