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Air leak,

If it`s a small leak the engine will idle high or if a big leak it won`t start or if it does it will run rough.

Knock sensor maybe

Wurth Bumper Dye GREY £17.28 inc P&P

You should be able to get it open even with the battery totally flat and deadlocked.
You have to pull the handle out slightly and work the key slowly back and forth and it should release the door...