Does it have a square stem? Do the leaves smell a bit minty when crushed?

reminds me of Lythrum [loosestrife] but the flowers are not usually singular.
How big are the flowers and is it the first year you have seen it or did it grow for you last year?

Maybe Epilobium parviflorum?

I thought of Lythrum as well, but it doesn't look quite right. It's not an Epilobium.

The four-petalled flowers are like some member of the Onagraceae, but which one is a mystery to me, now

Feverfew, I do think the stems are squarish but no minty smell.
Seaburngirl, I too thought it was a Lythrum the flowers are small and do go up the stem I only got the plant this year as a bargain from a supermarket, it looked sorry for itself, had no label but has settled in a treat but only just started flowering.
Tugbrethil I shall research Onagraceae.
Thanks all :)