It looks like a form of Blood-Leaf (Iresine).

I would say its Houttuynai cordata chemeleon - I hate it it invades everywhere including pond and smells! It took me 5 years to get rid of when it escaped from a pot.

Thanks Tugbrethil - Googling has come up with Iresine "Pink Icing" which looks most likely.
I thought Houttuynia originally but it isn't, no fishy smell and the leaf colouring is wrong - too much pink for all the Houttynia species I can think off.

It looks like what we used to call the 'traffic light' plant, sorry I don't know it's proper name. It spreads like wildfire, and takes ages to get rid of, by leaving just one tiny piece of root in the ground you get a new plant spreading.

If I remember correctly, Houttuynia has alternate leaves. These leaves are opposite.