andy:
Sounds like it is the temperatures at night that are
making your plant wilt. I have some tropicals that
show signs of cold stress in the 4-9 celsius. This plant
is used to being warm. If you put a frost blanket over it
at night see if that does not help. I have seen wilting
of plants at 10 celsius with a light wind just because
of wind chill.
Art

I would have thought that wet feet may be the problem. Last winter I kept my S. sagittata in the glasshouse, where the temperature fell to 2 degrees several nights. Sagittata was quite happy with this and flowered on and off throughout winter.

I second the combination of wet and cold feet.

Thanks for all that. I took some emergency cuttings,so hopefully save it over winter and choose a different position next season.