Just tested again and no spark when connecting alt output to battery + with engine off. Must have been residual current previously. I am going to check connections and cable (which is nearly 40 years old) but in principle it seems that I could run a second cable from the alternator directly to the battery (in addition to the original which went to the post with a spur to the battery). This might be belt and braces.I suggest the above pictures in the thread to make sure you are connecting the correct + of the alternator to + Batt.
When engine isn't running, there shouldn't be any spark whatsoever when you connect battery + to alternator + terminal.
Possible cause, short circuit somewhere , or a bad diode rectifier inside the alternator.