Thanks Suzy, that's a great thread. Since I started this thread, I've subsequently caved and got myself a dew heater + heating strips
My primary and secondary mirrors don't seem to dew up (short of leaving it pointing at zenith for a few hours...), but the eyepieces and finder dewing up was starting to really bug me
I also calculated that the disposable hand warmers I'd been using for wide field photography was poor value when shooting with multiple cameras/lenses/etc.