Calvin Sambrook :
I recall once camping in South Wales in December with a university
canoeing club where the butane cylinders we were using were only
producing a tiny flame. We balanced one on top of the other to heat it
up and make it useful...

Sounds familiar. I remember camping in Scotland and keeping the butane
cylinder in the bottom of my sleeping bag to ensure a hot cup of tea in
the morning. Even so it was a bit slow, so I placed the (large, hose-
connected) cylinder on top of the pan.

--
Mike Barnes