Mel, they are the ones I meant, wasn't sure of their correct name but those are the ones...just wasn't sure if the shroud was a necessary requirement or not.
I reckon that some sort of shroud is necessary to stop condensation forming on the plug ceramic and shorting out the spark on cold damp mornings.
Mine are rubber, but then my plugs are set in a recess in the head so don't need to be as long as the 2 litre plug shroud.
The metal shroud is probably there to add rigidity rather than suppression because the plugs have an integral suppressor resistor (as does any plug with an 'R' in its part number)