Good info, Tony.
Here in the States, we don't have experience with sound moderators (which we call "silencers"), as the ones for powderburners are highly controlled by federal law - on the level of machine guns!
Air guns in general are not popular here so we don't have much experience with airgun moderators. They would be legally "iffy" here. Airguns are not (generally) regarded as firearms and are virtually unregulated, so a moderator for an airgun wouldn't be regulated either, UNLESS it could be used on a powder firearm.
Notice I say if it "could" be used: possessing the silencer, even without a gun that it fit, would still be an offense.
To be on the safe side here, it would be best if the moderator were built integral with the air gun - if it couldn't be removed without destroying both.
Second-best would be if it were very flimsy and you could convince the Federal firearms people that it wouldn't work on a cartridge firearm.
But, I'm getting way off-topic here.
Jim