The convention is that MPs don't lie. But the convention is also that ministers who have unintentionally misled the House come back to correct the record (i.e. take the opportunity to say that what was said previously was not true). One MP has been thrown out for breaking one convention, but there is no sanction against a minister who has misled the House but will not come back to correct the record. So we need a new convention that ministers who have misled the House but will not put it right can be called out as the liars they are / he is.
Or at the least a mechanism whereby if a member says a minister misled the House, the Speaker can directly ask (e.g.) "You said this at last week's PMQ and it has been suggested that this was not an accurate statement; following a formal request by the member from ...., I ask the Prime Minister, 'Was that statement true?'"