[cc]

This was in a pot which should have produced tumbling Tom's!!

[Yorkie]

Hmm, not sure.  Is it in compost or soil?  It's obviously struggling for enough light.

[juvenal]

Tumbling Tom? It's his lanky sister, Towering Tina..

[cc]

In Compost.
Struggling for light because I am not sure I should give it some.

[missmoneypenny]

Looks like a volunteer potato to me.

[Yorkie]

Yes, it does missmoneypenny, you're right! But if it's shop bought compost then it can't be as that will have been sterilised.  Potatoes and tomatoes are the same family, though.   The photo isn't clear enough to show whether the leaves look like tomato leaves, but the stem of the seedling is quite thick - unlike what I'd expect of a tomato at this stage in growth.

CC, once you've got a seedling you must always give it proper light or it will become tall, weak and spindly.  I assume that you do mean that it's in shop bought compost (as opposed to home grown compost)?

I'm still not sure I know what the seedling is.  Any chance of a more detailed photo of the leaves?  Also, how long ago did you sow the seeds?

[missmoneypenny]

How about carefully removing it and brushing the soil off to see what’s at the base of the stalk? If it’s a volunteer there’ll be a little tuber there.