I recall a local meeting where a manager at Arriva spoke about the Temsa seats following a question. Apparently, due to DDA limits on how high a seat can be from the floor in inches, if thicker padding was added to the Temsa seats, it would go over the height limit and would not be DDA.
Structural changes are therefore required and that is why they have not done them yet.
RE: Arriva North East: Latest News & Discussion - July 2016