Lobster wrote:
People keep talking about heating for a few secs; well it's taking me
more like 1-1.5 minutes to get the joint up to enough temperature to
melt the solder. And all that time the flux is boiling/burning off I
think. But heated for less than that time, I push the solder against
the pipe and it *just* melts, forming a blob which immediately
solidifies without running up the fitting, because the pipework/fitting
isn't hot enough. I'm using an ordinary butane cartridge blowlamp, with
a brass burner approx 15mm diameter and which seems to give out a good
blast of flame, and I'm using the hot part of the flame, not the blue
unburnt area. Does this make any sense?

There's something wrong with your blow-lamp, unless you're just
wafting the flame around in the general vicinity of the work.
Angle the blowlamp along the pipe a bit so that the flame heats
more of it, don't go at 90 degrees to the pipe ("you're probably
already doing this" ). You should see the flux start to bubble
almost immediately, then see the cleaned copper, then the solder
should flow. Are you practicing on microbore, 15mm, or ?