Not for nothing did Private Eye nickname him the 'dirty digger' some years ago...
Yes, I believe they could be compelled to attend IF they remained in the country - and could be arrested in future if they fled for Contempt of Parliament so they would have to either attend or forever stay out of GB - though how much use their attendance will be is anyone's guess - I reckon they will just say 'No Comment' to most questions on the basis that it is 'sub judice'.

does that mean that inquirys parliamentary and public .... get dealt with after all legal situations are sorted? 

i heard some stuff on the radio about ... the point of public enquiry was to get to the truth that was beyond what a legal proceedings would be about focusing on a narrow margin of the law and enabling defendents and others to refuse to give evidence somehting like that... it was further suggested that parliamentary enquirys in this country were ill equipped to deal with the folks they were to call before them in contrast to the US who have invested staff and resources to make sure that the truth is revealed.. well that's what they said... shame we don't have these traditions but we have been pretty slow in getting freedom of information protocols to the fore... seem to recall that well it's taken the labour party 80? years while the tories after 400 + years haven't begun!  hmm

GOD BLESS!

Peace and Love,