If you cover the pitch overnight you never know what it is like until you take them off the next day, and you don't want to take them off too early if it is still freezing.
Portable heaters don't always work either, and you don't get much change from £1m for under soil heating at the lower end of the market.
If it get very cold there is little that can be done that is effective and affordable. Maybe a massive electric blanket could work
