Return to Demon's Tomb after solving it in main quest

[JK]

Howdy.

I've completed my second Great Circle run, and noticed that after skipping the end credits I'm offered to complete anything that wasn't done. I thought trying to collect all Ancient Relics I missed might be interesting, but there is at least one I apparently can't get to.

According to the map of Demon's Tomb (A Study in Fear), it's the relic at the far end. However, I can't get through a closed door and don't see any way around it. When opening the complete Demon's Tomb map, it's the large room at the bottom of the right page which floods.

Is it possible to get there again, or have I screwed myself out of the chance to get this done?

[Carighan]

I can still enter it. There's this side-path that you opened up when leaving it?

[JK]

I can get back into the tunnel system, but at the door of the sarcophagus room nothing goes. There is no switch, no anything I can see, just a door and behind that, as far as I'm aware, is a flooded room.