Mine is a 1963 3 litre MkII Coupe. I cannot promise this will apply to any other car. As usual the WSM is of limited value. This is a bench type seat with a central divider and arm rest pull down.
1) remove the seat bases, they just lifts straight up from the front off two locating pins each. 2) undo the four 7/16 nuts from the seat back, above the petrol tank. Not the central nut, that holds the tank in. 3) this is not mentioned anywhere else: there are two bolts low down in the middle of the seat back. Access to them is behind the central arm rest. Pull the arm down and work the back plate out. It is unsecured (in my car). And that is it.