What plant have I ended up with?

in Plants
Last year I sowed in to some plastic troughs and labelled the troughs. I won’t mention just yet what I think I sowed as I don’t want to influence the responses.
Last week I noticed 2 or 3 small plants growing in one of the troughs that were about 3 inched high. These plants that were growing are all the same. However, I am wondering if something has self sown in my troughs.
The photo here which I took with my phone less than 30 minutes ago is a close -up. So for perspective the leaf is about the size of a 2p coin. It is also quite a dark shade of green. Ignore the small greyish spec as they are just dried water splashes enlarged.

Last week I noticed 2 or 3 small plants growing in one of the troughs that were about 3 inched high. These plants that were growing are all the same. However, I am wondering if something has self sown in my troughs.
The photo here which I took with my phone less than 30 minutes ago is a close -up. So for perspective the leaf is about the size of a 2p coin. It is also quite a dark shade of green. Ignore the small greyish spec as they are just dried water splashes enlarged.
Hope someone recognises it.

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"You don't stop gardening because you get old, you get old because you stop gardening." - The Hampshire Hog
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No sign of any plumb stones germinating.
I have salvaged the beach trees though and potted them up. Shall grow them on until big enough to go in to Canklow woods-probably when they have filled out 2 litre sized pots.