So I was off pottering around doing some innocent dungeoneering when I came across someone claiming to be an Imperial Captain who pointed me into Forelhost. Having nothing better to do I accepted his challenge and cleared the dungeon, killing a dragon-priest and grabbing a mask on the way out. The game then told me hat I should kill the captain as he was an imposter (surprise!) Unfortunately I didn't overhear any conversation between the supposed captain and his storm-cloak side-kick due to an nasty attack of the dragons. When the dust settled the dragon, the captain and the anonymous storm-cloak soldier were all dead. On the captain's body was a note to the effect that he should obtain the dragon-priest mask and take it to the Labyrinthian, which brings me to my question.

I have previously cleared the Labyrinth. Does anyone know if there is any point to me flogging my way back there? Secondly, does anything about this have anything to do with the main quest or the civil war stuff, both of which I am trying to ignore?

You need to collect 8 total masks, before going back and one of those masks is tied to the MQ.

Thanks, Sean. I'll just park the mask and carry on dungeon crawling then.

You do know the masks can be stored in Labrynthian until you have all of them collected?
The eight masks will unlock a ninth mask, and the room that the masks are actually stored in is a safe place to horde loot in game, just do not loose the Wooden mask in Labrynthian.
If you have already found it and sold it, the vendor may still have it in thier inventory.