Description: Very invasive in the woods far from gardens

Description: This flowering shrub appears to flower every year in our garden. It borders our conservatory and seems to be quite hardy as we don't feed/water it throughout the year. The purplish/pink petals are quite small and only measure a millimetre or two across. The small light green leaves have a serrated edge to them. I would love to be able to give this plant a name? Please help me identify this flowering shrub! Is it common? Exotic? Natural to the UK? Many thanks in advance, SF.