My heating is set to 17 to 18 depending on the outside temperature (rented house with single glazing and heat loss rate increases in the cold), mrs usually has a fleece blanket over her and tshirt/sweater combo as she finds it cool, whilst i generally have on a t shirt and micro fleece top and normally more than comfortable, if it gets about 20, i'm usually stripping back to t shirt.
Duvet wise, I got (about 2 months ago) a wool duvet and it appears to have helped the duvet fight as it is warm enough for the mrs but still breathes enough not to overheat me, though i do sleep au natural, whilst mrs has pj's on, before (with both down and hollow fibre) one of us was always too warm or cold