What could this yellow flower be please?

I've had this growing in an overgrown spot among tall shrubs (privet hedge/conifer hedge/forsythia/young conker and some pink flowering bush ever since I moved in here.

It really brightens up a dull area, especially when the sun catches it, but I've never had any idea what it is, any ideas please?

 

Looks like one that came up in a thread the other day - Yellow loostrife.

 

Thank you!
Lysimachia vulgaris - Yellow Loosestrife - (Primulaceae) to give it's proper name, unrelated to the purple loosestrife and it likes it damp.

 

It looks more like L punctata to me.