Updates:

The government has now also referred material concerning this to the police. They’re also going to table legislation to strip Mandelson of his peerage which, given the size of the government’s majority and the fact that all the opposition parties apart from the Conservatives have demanded it, is more-or-less certain to pass.

Honestly, I don’t see how this is not a resigning matter for whoever signed off on ignoring security advice about Mandelson - up to and including the PM.