Hi Peter,
Pulled this from the interweb and may be of interest. Like Clive, good to see you over the weekend in the taxi!
Car batteries, like any electrical appliance, can pose a threat if handled improperly. They contain sulfuric acid and lead, and can explode.
Overcharging - Overcharging lead acid batteries can make the gases within the battery expand to the point where the battery becomes swollen, which can quickly turn into an explosion.
Sparks- Sparks near the exhaust vents through which the harmful buildup gases escape the battery can ignite them, causing the battery to explode
Underuse - Some lead acid batteries can build up gases if left unattended for a certain period of time, or if they fall into disrepair. This can lead to an explosion if a spark ignites the gases.
Flawed Battery - If the battery of a car being jumped has some flaw, the flow of electricity can cause sparks to ignite the gases of the dead battery.
Improper setup - If the battery is set inadequately in its base, or if the cables are improperly attached to the terminals, sparks may occur which ignite the gases emitted by the battery.