From that Guardian link:
Collins disputed the grounds for Nix’s refusal to appear and said the committee was considering using its power to summon him.
He said: “We have taken advice from the parliamentary authorities on this matter. He has not been charged with any offence. The subjudicary rules do not apply in these cases as there is no ongoing legal proceeding.”
The MP added: “We will be discussing this with the information commissioner on Thursday but it is certainly the intention of the committee to take this further and consider issuing a summons for Mr Nix to appear on a named day at some point in the near future.”