I actually got it wrong, impeding the progress of a player is an indirect freekick but holding a player, I.e. what Trossard, Rice, Saliba, Gabriel and Pablo were doing is an indirect freekick offence so if the only offence was Trossard holding Pablo, who then couldn't get his header off cleanly, then it would be a penalty.
I dont think they need to change the laws at all, just apply the rules as they are already written. It's in there in plain black and white;
"A direct free kick is awarded if a player commits any of the following offences:
a handball offence (except for the goalkeeper within their penalty area)
holds an opponent
impedes an opponent with contact
bites or spits at someone on the team lists or a match official
throws an object at the ball, an opponent or a match official, or makes contact with the ball with a held object"
I dont think asking referees to apply the Laws as they're written should be that controversial really, however, if they're going to use a 'loose' interpretation of the laws then they should use that same interpretation throughout the entire season.
The entire decision to ignore the laws at set plays isn't just 1 particularly easygoing referee ignoring it while the others played to the rules, nor was the decision to pull up the foul at the weekend down to a particularly strict referee as is evidenced by the other similar incidents in games they've referee'd in the last 2 seasons. All referees have different personalities and some are fussier than others, some easier to con with a dive, etc but the decision to ignore holding for the past two seasons by all referees in all games means this is the standard that they've been refereeing to and players have become accustomed to, adjusting tactics accordingly. To then choose the one time they apply the laws correctly in regard to 1 possible foul, while simultaneously ignoring the multiple fouls occurring before the goalkeeper is even touched, in the dying minutes of a game at the arse end of the season to rule out a goal that in all likelihood has now decided the league winners and the final relegated team is baffling beyond belief.
I genuinely believe that if this scenario happened in Italy fans of all clubs over here, Arsenal fans included, would be pretty convinced that it was just another Calciopoli scandal waiting to be broken.