I'd expect it'd be a faulty stat if it's bad enough to notice through heaters. I check my temp whilst driving in this weather (on scangauge) and it still happily stays around where it does normally, even when driving at very low revs. Also was doing 70+ on the m25 (also through the 50 zone, in 5th, with fuck all revs), with an intake temp of -6oC(was half 12 friday night), and the car stayed up to temperature. mines got the same engine variant, the se. Albeit in a bigger displacement.