What tool to tighten/untighten this reducer?

[Captain Scarlet]

Hi everyone

Sorry for asking a daft question. What tool do I need to untighten this from my shower?

TIA

 

[gmartine]

A large allen key, measure across flats for right size or buy a set.

 

[snowhead]

It's most likely a ½" radiator valve key (sometimes mis-described as 12mm), but check size first.

 

[Timmy D]

Tap spanner/box spanner works in a pinch.