Freeing seized spark plugs-hot or cold engine?

[vanordinaire]

2006 Fiesta 1.25 'duratec' engine, alloy head.
I tried to remove the plugs at the weekend but bent a cheap plug socket on the first one without moving it. I stopped and am ready to soak in wd40 overnight and try again with a decent plug socket and a heavy bar.
Question is , should I warm up the engine first or do it cold?

[vanordinaire]

Thanks for the advice, still not sure if I should do it with the engine hot or cold though? If it's hot , does the alloy expand and tighten round the plug or does the hole expand and slacken the plug? Also does being hot weaken the alloy and make it more likely to damage it if forced?