Further to Tee Gee's comprehensive reply, I endorse the fan heater recommendation.The 2kw rating is sufficient to react quickly to falling temperatures and the thermostat essential to prevent overheating. I have an early edition of this* and , despite spiders invading it(and sometimes bbq ing themselves!), it does the job.
*www.diy.com/departments/apollo-grey-electric-greenhouse-heater-2kw/5060031170672_BQ.prd
Please ensure that you prevent direct water entry (irrigation and condensation) and lift it above the floor, perhaps on bricks.