Weed in my garden, Hertfordshire

Weed in my garden, Hertfordshire

Observation - Weed in my garden, Hertfordshire - UK and Ireland. Description: Popped up two different places in my garden last month, but I've never seen it before. Very pale pink/purple flowers with yellow mark in centre. Leaves opposite and toothed. 6-8 inches tall (so far).

Popped up two different places in my garden last month, but I've never seen it before. Very pale pink/purple flowers with yellow mark in centre. Leaves opposite and toothed. 6-8 inches tall (so far).