Hope my previous remarks help, Nick. I quite agree some songs can work fine, whilst others are a struggle: it's still the same for me, after I-dread-admit-how-many years of singing! If you want to know a bit more singing, without getting too bogged down in theory and technique, a singing teacher friend swears by

How to Sing
by Graham Hewitt

(Not to be confused with the many other books called 'How to Sing'!) I think it's currently out of print, but Amazon list several second-hand & market-place copies