I understand that even plain flaps can contribute in the region of 50% extra lift but is this maximum delivered at lower flap deflections (20-30) or does it rise progressively?
I appreciated that drag overtakes lift but doe the lift decline anyway with increased settings?
It rises progressively. There would be no purpose in a flap setting which generated less lift than a lower angle did, even if it generated more drag.
Takeoff flap settings are generally designed to provide optimal L/D which generally means keeping the drag induced by the flap as low as possible.
Landing flap settings generally want to get as much lift as possible (for low approach speeds) and don't care TOO much about the drag going high, because drag on approach is usually a good thing.
But if I had a situation where, say, flaps 45 gave a nice high lift and highish drag for landing, and flaps 60 gave less maximum lift with more drag, the "flaps 60" landing would have to be conducted at higher speeds, so any benefit of the drag would be lost through having to land faster.