I'd suggest you stretch them 5-10min a day till they go down at 11-2 ish and bring bedtime forward to 6pm till they adjust. You might need to give them a bigger morning tea or early lunch before their nap.
This is what I did with my DD and I would also put her in the pram and take her for a walk in the afternoon around 3/3:30ish so she could have quiet time.
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