Technical Mylife spark plug change

[Mylife61]

I want to change the spark plugs on my Mylife (1.2 Euro 5 engine) and I can see I need take the air filter box off to gain access to them. What's the trick to undoing the single rear fixing? Am I right in thinking it needs 4mm allen key which is not too short and not too long.

Also any other tips which will save me scuffing my knuckles, other than getting someone else to do it, would be appreciated.

All the best

Peter