Oh poor you, that is chilly isn't it? I always thought the recommendations were a little on the light side, but I'm a cold person! My DD however seems to be a warm one like DH. So when she goes to bed, the house is around 21c and gets down to around 18c, sometimes colder and we just have her in pjs and a 2.5tog bag and she seems fine.
I wouldn't use a heater as you said, it dries the air out. I would've thought what you're doing is what I would dress DD in. I would check on her during the night tonight by trying to feel her back under the clothes if you can without disturbing her. Do remember body temperature drops when sleeping.
On the plus side, this nap time is looking good. What time did she wake this morning in the end?