Instructions are not clear are they.
From your picture you have three terminals I assume one on each side is an earth terminal?
From the instruction diagram there should be three live terminals on the unit but you have six.
Possibly a dual dual unit but unlikely. If one terminal on each unit is not an earth then you could have the junction points for every point on each switch coming out.
If you were to check continuity between terminals that should tell you which is C,1,2.
If there are two earth points then you ought to have only one contact on the left hand side according to the diagram.
However from the instructions stating that jumpers are supplied to minimise wiring and that you have a black and a green/yellow cable in the box I would expect that the far left terminal is C, middle is earth, next terminal is a "jumper" (black cable) from left hand unit to the left hand terminal on the right hand unit, middle earth again and lastly 1 on the far right.
Supply into C and switched line out from 1 to heater. Do not know where terminal 2 has gone!
added a piccy since I had done it, I think the wiring is as the lower picture.