Couldn't find this thread till i was pointed in the right direction so i'll re-post here:
This doesn't appear to have been mentioned on F1T but according to DeepThroat (an RBR mechanic) over at
the RBR forum, it was a seized rear centre spring that effectively stiffened the rear of Marks car almost to the stage of being un-drivable hence his lack of pace compared to Seb and significantly worse rear tire wear.
After reading DeepThroats comments it sounds to me like the Inerter* (basically works like spinning top toy in a tube providing a Tuned mass damper style effect without being illegal as the "mass" is fixed in place and spins instead of moving backwards and forwards) failed as it's the most like component to "seize up" in this scenario.
It's pi$$ poor that RBR didn't find and fix this before qually!
*sometimes referred to as the 3rd spring