In Lair of Infinite Agony you will see there are 3 plates of crumpets to collect.
This first crumpets that are easily found, post main story, are in the Wailer's Drop. Take the elevator up from the bottom of the Drop and head over to the pit you just fell through. Look along the northern wall to find a small opening where you can collect these crumpets. If you want a good marker of where to drop, look along the lip of the drop until you find 2 chests next to each other by the pillars framing the Drop.
This second plate of crumpets are to the East from the first. Go back down to the bottom of Wailer's Drop. Head to the east, to the far elevator. Look along the Eastern wall as you go up to find this set of crumpets. They are right by the elevator so you can ride it up and down until you either jump and land on the pillar with them or can just grab them from the lift.
The third plate of crumpets are found in . From the second set of Crumpets, head to the main elevator that brings you up along Wailer's Drop. Go North intoHall of the Dead. Run to the lip and jump down into Death Quencher Well This can also be access through Crawler Hall and crossing over to the far side of Death Quencher Well, climb up the ladder to the Crumpets.
You will find the third plate of Crumpets just sitting on the ruins of the bridge.