Broken Kode Discussion Forums

Hi there, thanks for reading.

I'm using the Rin theme which i've modified myself in a couple of places. Everything worked fine for about 15 postings, but now suddenly the layout has changed when i view in the single-post page. Would it be possible for someone to point me in the right direction to fix it, ie do i need to change the css file (if so where?) or is this a php problem. To view the problem please go to http://www.lightscape-images.com/wordpress
If you go to the single view (of the postings) you will see the problem. Now go back about 3 postings and you will see how it should be. Note i hadn't done anything to the css during this time. Really grateful for any help.
Best wishes Neil