There's a remarkable passage in the clerk's letter which should give people a sense of the overreach and what's really at issue here:
It's line-crossing in a constitutional sense. And it's intimidatory in a political sense. A committee can seek documents to assist it in the legitimate work it does, legislating, holding the government to account, etc. But not for the making of legal determinations.
And finally, note what the clerk says in conclusion, in terms of big picture considerations:
"I feel that the … principle of the separation of powers … is sufficiently important in constitutional terms that a court might see the merits of the argument and rule against the House," he writes.Over to you, Conservatives...
"In my view, respect for the constitutional framework of our parliamentary system of government is part of the rule of law which is the over-riding legal principle that makes a democratic system of government such as ours workable and credible."