Clive,
Think very carefully before putting a flame to it. When we had to repair tanks in the workshops, they were hot washed or steam cleaned for sometime, then left for a period and a special meter used to "sniff" inside. The boss may have been soldered originally, so brazing may not be a good idea. If it is cleaned well around the crack, solder is best solution.
I actually use a radiator repair man for my tanks, they often do not advertise, but are well known in the local motor trade. Worth getting someone like that to do it.