Solved I think this is sea beet

This came up in my raised bed this year. I think the seed may have come in with the seaweed I put around the cabbages last winter. Do you agree it's sea beet?

 

It does look like it. See if the following identification guide for Sea Beet helps:
https://www.foragingcoursecompany.co.uk/post/foraging-guide-sea-beet

 

Cultivated beets have a resistence to salt water; sugar beet are the only crop for which salt use as a weedkiller is approved in UK.Mainly used to kill volunteer potatoes .

 

Interesting! I think it's quite a handsome plant. I thought I would save the seed off it and use them for microgreens.

 

The link is very useful. The striped stems clinch it. Incidentally the Huauzontle behind it which is in the same family also has red striped stems. Neither seem to be bothered much with pests.