Wordpress site keeps being defaced/hacked

Ok i dont know whats going on but i think someone has inserted some backdoor code into my wordpress installation and keep defacing my index/home page. I need someone to find the hole and sort it out for me so he is unable to carry on performing this hack on my website. It whould be a quick job for someone who knows what they are doing and looking for. It only seems to affect the home page and nothing else the admin area etc are all fine.