There is a plant in my garden which I haven't planted. It spreads like crazy, which makes
me think that it's a weed. It sends up runners all over the place, rather like a
buttercup weed and it has long roots like a dandelion. However, it looks very much like a
hardy geranium, and it has purple flowers in summer.

If someone could identify the plant, I've scanned in its image on this website:

http://www.v-b-elec.u-net.com/plant/plant.htm

Many thanks.