It's probably a Thorn apple (Datura stramonium) which does suddenly appear from nowhere! Everyone seems paranoid about it as it is poisonous like a huge variety of plants in the garden. If it has a prickly seed husk like a Horse Chestnut, then that's what it is.
Personally I like them and provided you haven't children or dogs and are not in the habit of eating bits off your plants, can't see a problem adding it to a wild flower meadow. No doubt you will get lots of replies!

Funny how true that is; half the plants in our garden are poisonous. Some plants seem to get more bad publicity than others.

Its Nicandra physalodes, common name Shoo fly plant - the seeds from this are often a component of bird seed mixes, and random plants occasionally pop up on their own, probably a present from the birds.

If it produces seeds, you can certainly try collecting them to get more plants next year, otherwise you can buy packets of seeds. The plant isn't edible, but is not known to be particularly harmful, more info below

https://www.rhs.org.uk/advice/profile?pid=538

Yes, I thought it was shoo-fly too: very recognisable seed cases and a pretty blue flower. Easy from seed :)

Given the name 'Shoo fly', associated with it's insecticidal properties, it may not be ideal for a bug-friendly meadow area?

Well, its not proven it has insecticidal properties, although there is anecdotal evidence that it repels white fly - no bad thing in my book!

Thanks for the information, I have been away on holiday and not able to reply before now, yes I think you are right it is a 'shoo fly' plant. I will collect some of the seeds and see how I get on.