Why only "in theory"?
Because the last time any non-Member of Parliament was actually imprisoned for contempt of Parliament was 1880.
"Basically, it never happens," says Béar. In 1999, he continues, a House of Commons committee recommended that Parliament abolish its power to imprison non-MPs and replace it with the power to fine them -- a sanction that the body might feel more comfortable exercising. But the recommendation was never acted upon, so Parliament's only power to enforce a contempt order -- other than by symbolic reprimand -- remains the one that, in modern times, it never resorts to. (The last major stand-off of this type, according to Lavender and Béar, involved, ironically, the sons of Murdoch's late rival publisher, Robert Maxwell, who ran the Mirror Group of newspapers. Ian and Kevin Maxwell refused to answer a select committee's questions in 1992 about alleged misappropriation of company pension funds, but they suffered no real punishment other than public humiliation.)
"There is a convention that Parliament respects the judiciary," says Béar. "MPs accept in theory that they shouldn't encroach unnecessarily on the workings of the judicial and criminal justice system." There is also recognition that law enforcement officials leading criminal investigations would prefer that politicians butt-out to the extent possible, so that matters can proceed logically and methodically, rather than being subject to tactically unwise decisions motivated by emotion, impulse, politics, or grandstanding.
Finally, there is some real danger that Parliament's questions could legally derail later cases that might be brought, even against defendants other than these witnesses.
"There is a risk," says Béar, "that somebody would be compelled to answer a question which they couldn't have been compelled to answer at a police inquiry or in a court of law" because of Parliament's power to override evidentiary privileges. That fact "might later be relied upon as a basis for having the criminal trial stopped," he continues, on the theory that the charges were "unfairly brought," or that they involved an "abuse of process." Even though Parliamentary testimony can't later be admitted into evidence, it can lead investigators to other evidence, which defense lawyers might later argue was tainted by the extrajudicial means used to unearth it.
A very rough analogy for U.S. readers might be what happened to the prosecution of Colonel Oliver North, who was forced, under a grant of immunity, to testify in Congressional hearings concerning the Iran/Contra affair in 1987. Later, North was convicted of criminal charges stemming from the affair, but his conviction was thrown out on appeal in 1990, in part because his trial might have been indirectly tainted by the forced testimony he gave before Congress, even though the testimony itself was never introduced against him.
What's the bottom line then? What should we expect Tuesday?
Nobody is apt to end up in shackles under Big Ben. Expect, rather, a "loose and unsatisfying discussion," Béar predicts, at which the witnesses will "take a cautious line on what information could undermine other investigations."
The committee, in turn, will probably raucously condemn the witnesses' vague and oblique responses but, in the end, acquiesce in them.
:o)
And it couldn't be happening to finer fellow. 
