If anyone encounters this problem in future, removing the brake lever is the way to loosen the clamp but my clamp (as suggested by Hilly Ben) was a bit stuck fast and after a bit more wiggling and coaxing I could move it.
@Millieri started
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If anyone encounters this problem in future, removing the brake lever is the way to loosen the clamp but my clamp (as suggested by Hilly Ben) was a bit stuck fast and after a bit more wiggling and coaxing I could move it.