As i see it you are being accused of negligence so it is up to the accuser to prove this and if all you did is change the oil and filter they are going to have a hard job doing so.
Have your day in court and stand your ground.
By the way I have just priced a DME (ECU) for a 2007 BMW 320i from the BMW agent and it comes in at just over £900, so where the Citroen agent gets there price from is anyone's guess.
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