Struggling with this seashore plant, help needed please

[BILLy]

Hi all
Been doing some coastal flora and fauna research and came across one I'm not familiar with, and would like to ask for some help with its ID please
When I crushed the leaf it spelt very peppery and mustardy. The pictures should tell you more
Cheers
Bill

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[Tengu]

I am the last person to ask about plants but that reminds me of radish.