Please could you tell me what this is?!

[hollandnigel]

 

[Esther Knapicius]

need a further away shot.

 

[Tetters]

Hello @hollandnigel , and welcome to the forums . I'm not 100 % sure, but that reminds me of water cress. Is it growing in water?

 

[hollandnigel]

No, it's in a pot that I had chilli plants in,

I'm based in Spain if that makes a difference

 

[roadrunner]

Reminds me of chickweed https://en.wikipedia.org/wiki/Stellaria_media

 

[hollandnigel]

Here's a shot further away,

There are some chili seeds in there so didn't want to pull it all up incase I pull them out too.

But guessing this could stop the seeds from growing??

 

[Sheal]

It looks like Chickweed and will have tiny white flowers.

 

[hollandnigel]

And it'll dominate the pot and prevent anything else growing I assume?

 

[roadrunner]

And it'll dominate the pot and prevent anything else growing I assume?

Yes, it will basically create an impenetrable barrier for any seedlings, on the plus side chickweed is edible, but if you get it in your garden, you'll always be fighting it, because it reproduces not only by countless seeds, but also rhizomes.

If you look at your first picture in your opening post, you can see a reddish stem coming out of the mass, which is where the flower was and now it is forming seeds. Soon all those plants will be producing those "stems" and that's what really forms the impenetrable barrier for seedlings, because they will get all tangled together and form that barrier.