RateTheRef

See this is what I thought however when reading the LOTG I thought that it's possibly a IDFK from the touchline where the offence took place?

Law 12.4 seems to be the nearest to deal with the situation you cite, although it only refers to officials OUTSIDE the field of play.

"If, when the ball is in play..............a substitute, substituted or sent-off player, or team official commits an offence against, or interferes with, an opposing player or a match official outside the field of play, play is restarted with a free kick on the boundary line nearest to where the offence/interference occurred; for direct free kick offences, a penalty kick is awarded if this is within the offender’s penalty area."

Alternatively, as I suggested earlier, I'd wait until the ball next goes out of play or play is stopped for another reason, deal with it and restart accordingly.