TomO this is what works for me.
Once the seat is lifted I push the retaining hook (of the seat) completly in and than pull my finger away so the hook "springs" back up. For the passengers side do this with your right hand pulling the back of the seat at the same time
with your left hand into the forward direction.
This works 9 times on the 10 for me.
There is a second latch for the back ( Lh and Rh) between the door seal and the latch of the door hook. It is a small black button you can push in to unlock the back. I have never seen one working and of course I tried to find out why. It's easy the pull back the door seal and lift up the uphostery.
Than you will see the most clumsy and cheapest part of the W 124 T.
I simply don't beleave it ever worked, not even when new.
So fiddling to it will not help. There is an interconnecting between de seat latch and the back latch with so mutch play it will NEVER work!
But let the seat latch jumping up happely disengage the back lock enough.