AHA!!! I've sussed it out! I accidently deleted the } at the end of the container in my css stylesheet!
Nearly there folks!
The next bit is... the div now stretch to 3870px wide which isn't what I want as it leaves a load of white space after my last image. 3870px is the maximum I need it to stretch with it being an external style sheet. This style sheet will be used for about 10 html page/projects. Is there anyway of making the 3870px a maximum so that the div automatically stretch?