The extra £80 for a referb from Dell is not such a bad thing, as AK says you will get some sort of warranty.
If your happy with your other components then a motherboard bundle is also a viable option. Have a look here for motherboard bundles and bare bone systems......
http://www.novatech.co.uk/products/motherboardbundles/mbb-i5240a.html
http://www.novatech.co.uk/products/barebones/bb-23204a.html