I usually reckon to have a small blower heater on a thermostat, which kicks in just above freezing point.
At this time of year, we just have over-wintering geraniums, fuscias, cuttings etc, so they'll usually survive, but as for sowing in the late winter, it takes a little more care to ensure the trays are kept warm/covered a night, or more often than not, germinated in the house first.
Chillis tend to give up around now, so we tend to start new ones each year from seed!