I don't use bivvy heaters, electric blankets, hot water bottles, groundsheets, and I use a brolly all winter. The secret is decent togs and a decent bag - I mean really decent - spend some money. Bass layer, micro fleece, hoody (sometimes), but the insulated jacket/salopettes with down/softie filling are what matters. Same goes for sleeping bag - pay the price to get real quality.
The Snugpak stuff is a good place to start and not that pricey compared to some of the other outdoorsy stuff (Rab etc). Avoid anything from fishing brands - most of it is overpriced tat.