One of the first caterpillars I've seen this year.
I think It's a Large White Butterfly Toadflax Moth or Toadflax Brocade Moth offspring munching it's way through Purple Toadflax seedheads.  Thanks to gen2 who has identified it for me 
Luna and I have entertained 'the boys' today, but there wasn't much chasing balls in the 27 degrees sunshine.  We've all had a calm and relaxing day.