Thanks esmi.
I tried that. No luck, unfortunately.
I'm starting to think the database has some sort of error. It now doesn't seem to matter which theme I'm using, the site has these problems.
I have also just created a test page. It is formatted incorrectly (compared to the perfectly fine home page).
http://www.davidhansard.net/test-page/
hmmm.
I suppose the problem in my solving this is also related to the fact that I'm new to Wordpress - and don't actually know where to look for the html file which lays behind say the test page.