Rather than putting heat into the system (which is expensive) think about how you can minimise heat loss from the system.
First of all, cover up the windscreen with something.
Pop along to your local charity shops and buy up as many blankets, heavy curtains, towels and tatty fur coats as you can get your hands on. Use these to wrap up your tank and pipes etc - and especially the pump.
Pour some warm water into the tank before going to bed. It should stay above freezing all night as long as the insulation is good enough.