On www.classiccarsforsale.co.uk there's currently a black 2007 Arnage advertised. There are many photos including the trunk floor and spare tyre well underneath.
I don't know but on the Bentley Heritage site under Arnage Luggage Compartment there's a diagram that shows the spare wheel with lifting strap and a spare wheel cover. It seems that the spare wheel just sits in the wheel well and the lifting strap is used to lift it out. Have a look at their diagram, there's no mention of anything under the wheel except the lower side of the lifting strap.