Found this tip on the internet and it worked like a dream for me this morning. Key had broken of flush with the lock face and couldn't grab it with thin nosed pliers or tweezers. So......
Get a fretsaw blade*, hold with teath pointing towards you, snap off blank bit at far end (so that teeth go right to the end), slide in alongside key (5mm was enough), gently twist so that the teeth bite into the broken blade, pull out carefully untill enough of blade visible to grap with pliers.
- tried first with a coping saw blade - but too thick to fit alongside the key blade.