Loosestrife

Or to give it its full name Lysimachia punctata.  It's growing in abundance in the wild garden just now, but will have to watch it doesn't try to take over.  On a dull day like today, it certainly adds a bit of brightness, along with the marsh marigold round the ponds.

The rabbits in the park seem to have multiplied recently too (see Extra) as there are lots of little ones around.