It doesn't seem possible to order the correct 2.5" mounting screws and grommets from HP (although they appear in the parts store they don't seem to be orderable). So I made up equivalents using a couple of Maplin's rubber vibration washers and a large headed bolt.
Here's a link to the washers (pack of 50) : http://www.maplin.co.uk/p/anti-vibration-washers-a26gf