I've looked and looked and can't work out where I change the settings for this -
on www.johnnycajon.com I want the black horizontal bar under the menu (and above the footer) to reach the black border around the whole container.

Any help much appreciated!

You have a 10px padding is set on the container - ATO->Style & configure LAYOUT->Layout Container Style

doh! thank you, it says it only applies to top and bottom padding so I ignored it.
Now I've fiddled around with the site so much I can't work out how to get more space around the posts!

Remember you have a header, middle and footer. In the middle you have the left sidebar(s) the center area and the right sidebar(s). In the center area go the posts.

So you could look at the center area options and/or the styling for the post/page options

Yep, did that, I have 20px set on the center column and 10px on the post/page. It's fine in the actual post or page, it's only on the first page it's happening. I checked excerpts and I don't see anything there I can change.

Thank you!

I'm not sure I understand what the problem is now. I see lots of left and right padding on the home page

edit: you have a caching plugin active....flush the cache and turn it off when doing changes.

I think my brain has fallen out, caching plugin! of course. Thank you again! All ok now.