I'm thinking that the bubble wrap suggestion was a great idea!
I keep my main heater down in the low 60's and use a portable electric heater in the my bedroom if I need it. I've got one that's pretty efficient for the room size but I can see it on my my power bill.
I doubt if there's anyone here that's as much of a cheap skate as I am but if I'm too cold to relax, that's no good.
Anyway, it sounds like your ancient heater system works okay most of the time so you might try buying something for these crazy cold nights just to keep the peace?