Thanks Crossref. That's where I'd seen it. Strange that it's marked as a "clarification" but it isn't in the clarification section of WR's website, even though it's one that most of us will come across (and can easily get caught out by) regularly.
IRB (sic) clarification from September 2014
IRB clarification for teams choosing not to engage at the lineout
• if the defenders in the line out choose to not engage the line out drive by leaving the line out as a group, PK to attacking team;
• if the defenders in the line out choose to not engage the line out drive by simply opening up a gap and creating space and not leaving the line out, the following process would be followed:
- attackers would need to keep the ball with the front player, if they were to drive down-field (therefore play on, general play - defenders could either engage to form a maul, or tackle the ball carrier only);
- if they had immediately passed it back to the player at the rear of the group, the referee would tell them to use it which they must do immediately...
- if they drove forward with the ball at the back (did not release the ball), the referee would award a scrum for accidental offside rather than PK for obstruction.