Just scanned the site with sucuri.net and it is reported clean (all green!)
The site is maximumdefinition.com if that's useful.
Are you suggesting I delete all the wp files on the server and re-install wp to remove all possible back-doors?
If I check all themes and plugins, what exactly am I looking for? I'm not a techie, I'm a self-taught WP user with no coding experience! Playing with html, and css to a lesser degree, isn't frightening, but I don't have any experience with PHP.
Can you recommend security plugins to minimize risks of being hacked again. I currently have :
Better WP security
Front end users
Limit login attempts (which has caught a number of attempts to login as admin)
Login security solution
User role editor
Thanks again for the support