Would you let it spread or not ?

This looks like Vicia cracca, the Tufted Vetch or Cow Vetch. The seed was probably passed by a bird. If you want to keep it, make sure it doesn't grow near any vegetable crops, as the roots contain toxins.

if its very tall it could be galega.

It's very similar to Vicia but having now looked up Galega the flowers and leaves match that perfectly. It's 2 or 3 feet tall at the moment. I see it's also called Goat's Rue and that its best scrambling through other plants for support which is exactly what its doing.

It will seed about!

i took a photo of mine today and i will load it later. but i do like it and it seeds freely. i have potted a few seedlings up for future cleared places.

I like it too. It is good for beneficial insects, but the purists will tell you it is a weed (I am assuming it is Galega officianalis). There are other 'smarter' galegas, G. orientalis for example. I find that if the soil is too rich, it really flops about. Best to give it a bit of a hard time!

purists what do they know pha. i love it and as definitions go 'a weed is a plant in the wrong place'. my dad said lupins were weeds coz he didnt like them. i have lots of british natives[wildflowers] that my lovely neighbours tell me are weeds. they are gorgeous.

Good for you! Me too. My mother in law baulks at valerian, but I love it! I'm with your Dad on lupins though; never liked them since I grew some in a previous garden and they were covered in lupin aphids, which, as I don't spray, were impossible to get rid of and I am now off them for life! Sorry!

i wish i could get valerian to take. it never survives for me. i have loads of lupins aphids but i dont care, as the blue tits feed on some of them.