Mandelson: in the Cabinet and outside Parliament?

At the risk of constitutional fogeyism the quesion arises: Has Mandelson actually started his new job? Is he - as of now - Secretary of State for Business? Because, as things stand, not only is he not an MP but he is not - yet - a member of the Lords (I suppose Lord Mandelson of Spin will emerge in due course). When is the last time a member of the Cabinet was outside Parliament? My guess is 1963. After Douglas-Home removed himself from the Lords but before he returned to the Commons, there was, I think, a - short - interval when the PM was outside Parliament. There were, I believe, letters to The Times questioning the constitutionality of this. The same question arises vis-a-vis Mandelson. The British Constitution is a mysterious thing, but is there not at least a convention that a Cabinet member should be in Parliament and shouldn't Mandelson wait until he is before assuming his duties? For that matter, once he is installed in the Lords, as I assume he will be, he will be a Secretary of State who is outside the Commons. There are precedents for that (leaving aside the Leader of the Lords and the Lord Chancelllor), most recently - I think - Charlie Falconer (for a few weeks Justice Secretary), but before that one has, unless I am mistaken, to go back to the 1980's (Lords Carrington and Young). Mandelson's position will be fairly unusual - he will be able to avoid being questioned in the Commons, not that he will be bothered about that.