Unknown caterpillar

Unknown caterpillar

Observation - Unknown caterpillar - UK and Ireland. Description: I saw this caterpillar walking across the capping stones on a wall in my garden a few weeks ago - it was about an inch long. I can't find this in my field guide - does anyone know what it is?

I saw this caterpillar walking across the capping stones on a wall in my garden a few weeks ago - it was about an inch long. I can't find this in my field guide - does anyone know what it is?