Betty Swollocks is bang on about shaving the plastic. I had a good look around my dodgy cap spurred on by his information and found some tiny rub marks on a piece of the housing right above the actuator. This aligned with a little ridge that I could see on the flap's catch. I shaved off about 0.5mm with a stanley knife and the flap opens really smoothly now. It did get worse initially when I hadn't quite shaved enough off to stop it rubbing but I had roughened up the existing surface, then kept going until it was all good. Due to it being dark outside I have only got a low res picture, see below for the piece I trimmed marked red.
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