Strictly, you may be right - it is not a criminal offence to ignore an invitation form a select committee. I believe (but may be wrong), that the committee can

Strictly, you may be right - it is not a criminal offence to ignore an invitation form a select committee. I believe (but may be wrong), that the committee can request Parliament to consider the matter, and at that point the Sargent at Arms (who does have legal powers) can fetch you to appear at the bar (entrance to) the House of Commons chamber. Practically speaking, no-one in their right mind, based in the UK, ignores an invitation to attend a Select Committee hearing. Very bad form :-)