Well three silver soldering jobs and much pickling later, I flushed out all the scale and put the burner back together. I filled the tank until the gauge was reading half full and then lit the fire.
There was a bit of a leak at the valve spindle, not too much of a surprise there. But rot my socks if there wasn't a tiny little flame joyfully poking out of one of the vaporising tubes below my silver soldered joint - the pipes are really shot - but I'll patch them up. This little fellow had not shown himself in my blowing into the burner with fingers over the holes while it was under water - fun yesterday. It's back to the brazing hearth tomorrow.......