Hi.
A few minor things.
1. The prices on the pages 'whats new' and 'specials' are not center aligned to the product boxes, to fix this remove width:100% form the div
ul#product_list li div.right_block or set width to width:auto;
The 'Add to cart' and 'view button create the width for the container so this width is not needed.
2.The product boxes could do with a little whitespace between them, it's not easy on the eye to view the products.
3. In the subcatagory boxes on the top of some of the pages the text is flowing out of the boxes (
http://www.northernarbsupplies.co.uk/190-karabiners) this is because a set height has been put on the boxes.
To fix this remove the height from
body#category #subcategories li and I would increase the height on
.greybox-cats li to 60px to compensate for longer titles.
4. IMO the green hover state on the navigation doesn't look very nice, nor the red hover state on the dropdown navigation, but thats aethsetics.
Other than these small points I think the site looks great and very comprehensive.
Good luck with it.
D