Thank you very much @Silver surfer and @Alan Clark2 in Liverpool would you say I can keep cutting it back for it to flower again or does it just go to seed? it says biennial on the details you sent over silver surfer so perhaps it will seed later this year once I stop cutting back and have lots of little plants this year that will flower next year? I'm ashamed to say I'm not sure I like it that much as it looks like a weed and I thought maybe calling it a white aster might make me like it more. Oh dear i'm sounding like a dreadful plant snob! But if it helps the bees etc that must be good!
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Looks like Erigeron annuus.
https://en.wikipedia.org/wiki/Erigeron_annuus
http://science.halleyhosting.com/nature/gorge/sun/daisy/erigeron/annuus.htm