Thanks for the suggestions. I'm avoiding approaches that use mains power, batteries or gas cannisters. Susan's garden sprayer approach sounded interesting so I thought I'd try it.

It basically worked. It would work better if it had a spray attachment designed for the job. Its capacity was only 5 litres, which is small compared to the 20l that the black bags offer, but I found it adequate at the low flow rates I was using. My kettle is 1.5l, and I found that 1 kettle of cold water and 2 of boiling gave around 60C, which was adequate (in fact too hot if the water wasn't in a fine spray).