Hiya guys,
thankyou for your replies. I could see what you were saying, but would require a lot of legwork and mean that I could not have on individual page with no sidebar (I think..)
Anyway, I did some digging through functions.php and found, a few lines from the end:
if ( is_singular() && ! is_home() && ! is_page_template( 'showcase.php' ) && ! is_page_template( 'sidebar-page.php' ) )
$classes[] = 'singular';
this set me in the right direction, however when I tried redeclaring that line, including single pages/page.php, I got a bunch of scary “fatal errors”.
I just found Someone else’s child theme to fix this issue (from this thread). Incorporated those functions and two pages into my child theme, works like a dream. Turns out you need to declare the single.php file, but under a different function.
Thankyou all once more for your help!