Having upgraded from Summer Beta to 1.8.2 I've reworked my skin main.php customisation from the CUSTOM SKIN

I get this error

Fatal error: Call to undefined function: skin_content_header() in /home/stmedia/public_html/skins/general2/_main.php on line 41

the line in question is

skin_content_header();	// Sets charset!

In my main.php it is identical to teh CUSTOM sking main.php but whilst the custom skin works mine doesn't. I must be missing something. Any ideas?