Shopping cart problems on front page

Im sure there's probably a simple answer to this but after 2 hours of trying different options to fix this I have ran out of ideas.

My shopping cart works fine on all pages apart from my home page, the home page is set up as a category listing. When i click proceed to checkout it just leaves me on the front page and adds /index.php/checkout at the end of the address.

Also product links in the cart dont work,you click on them and the page title changes, but I remain on the front page.

Any ideas would be much appreciated, thanks

here it is www.scorpiondirect.com