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'.