Import and customs charges
 

Does any body have any idea of duty and charges for importing engine parts from the good old US of A?
I'm looking at buying some engine parts, total cost is $600, but I can't make head nor tail of HM Revenue and Customs website. The FAQ just seem to cause additional confusion.
My understanding is anything upto £290 is charged at 3.5% plus the VAT at 17.5%, it doesn't say what the charges are for anything over £290.
Anybody got any experience of this?
Cheers.