I have noticed that for boosted applications it is recommended to use a 12mm manifold insulator with 25mm plenum spacer but for my VS 4.2L I was advised to buy each of the manifold insulator and plenum spacers, is there a reason it wasn’t recommended the

The reason why recommend, the 25mm plenum spacer and 12mm manifold insulator, is purely due to packaging reasons (space under the bonnet). Ideally you would want to go a 25mm manifold insulator and a 25mm plenum spacer, but would require the bonnet reinforcement to be reworked. I don’t know. Would have a copy of the correspondence that suggests this?