Freelander 2 Air Con question

[weldingwizard]

On another post on the forum I was topping up the air con on a Ford Focus.(First time) I would like to check the air con gas in a Freelander 2. I have located the Low inlet just below the windscreen but the inlet cap seems very tight. It looks like a large nut that has been plastic shrink wrapped. I am unsure if this is a cover to a nut or some odd push fit connector. I don't want to force it with a spanner and snap the end off. Any advice how to remove the cap so I can attach the air con gas gauge. Thanks

[weldingwizard]

This is the inlet of the cap I am trying to remove

[weldingwizard]

Still not musted up the courage to try and remove this. I presume it should not need a lot of pressure to remove? Just wondered if it requires a particular tool to get the cap off?.