Have a look at
Dealing With In-Flight Lithium Battery Fires In Portable Electronic Devices from the CAA. They used a blowtorch to ignite the battery.
It seems as though the greater risk is from spare batteries carried in baggage, which may be shorted out by a metallic object (eg a zip) across the contacts. There's a thread in Tech Log
Torch Batteries - Safety warning that mentions some restrictions on how much Lithium you can carry in spare batteries - hands up everyone who knows the Lithium content of these items!