I don't manage to find a match, the shape of the flowers do look like from Rocket Salad (Eruca vesicaria) but color and the leaves don't match..

Myriam, try this one -
Sinapis alba L. white mustard, http://plants.usda.gov/java/pr...

Thanks Chelle!
I also thought White Mustard because of the leaves but the shape of the flower petals don't really match mine..
http://www.google.be/imgres?im...

I kind of figured that you'd likely checked that one already, but it was worth a shot.

I seems that the Brassicaceae family crosses very easily, so mine might be one of these natural unnameable hybrids..

That thought had crossed my mind...

Brassica napus or Oilseed Rape comes close ..

http://en.wikipedia.org/wiki/R...

http://www.dgsgardening.btinte...

http://www.sciencephoto.com/me...

It's difficult to find many photos which show it with leaves and flowers which look the same, maybe it's variable?

Thanks Janet, yes Brassica napus does look close, but I still don't feel that the flowers are an exact match, apart from having longer flower petals and having a narrower base, I also do see subtle veins I don't see in the flowers of the Oilseed Rape..

Check out Raphanus raphanistrum ..

http://www.missouriplants.com/...

http://www.google.co.uk/search...

you have hit the nail, Janet, that's definitely the one!!! Thanks a lot!

Good going, Janet!

I found it by searching Brassica napus which came up on the Missouri plants site, then I looked at all the flowers in the 'yellow' group where I found it!

I once found the best (identifying) picture of a tree on an edibles website...I think it might have been a Beech tree. You just follow the breadcrumb trail, sometimes.