Question : what is the proper way to clean battery
terminals and leads that aren't making a good
connection?

Last week, after charging my battery when it was
flat, I noticed a lot of that bluish-white salt stuff
built up in the negative battery lead. I cleaned it
off and reattached the lead, and then as I was
doing up the nut with the spanner, I got a massive
spark discharge and burning bits of stuff flew off.
I take this to mean that the lead is not making a
good connection to the terminal, and the spanner
caused a connection when it must have come
into contact with the terminal properly.