SMALL BLUE FLOWERS - WHAT?

SMALL BLUE FLOWERS - WHAT?

Observation - SMALL BLUE FLOWERS - WHAT? - UK and Ireland. Description: This trailing plant is very common. It grows in neglected walls. The leaves are about a centimetre across - the flowers about the same. There are very few months in the year when it is not in flower. What is it? (It's embarrassing that I don't know!)

This trailing plant is very common. It grows in neglected walls. The leaves are about a centimetre across - the flowers about the same. There are very few months in the year when it is not in flower.

What is it? (It's embarrassing that I don't know!)

I've said the habitat is gardens and parks - but it isn't really. It's more in walls along the sides of roads, often in tatty places.