Can anyone help identify a caterpillar?

I want to ask about a caterpillar seen in Essex on a spindly but unidentified plant. Not hairy. Maybe 1.5 inches long. Rows of black blobs on a yellow background, but also grey on the flanks and on the legs. Definitely not box tree moth caterpillars, which which I'm sadly all too familiar. Similar to the mullein moth caterpillar, but definitely different. 

I have photos but couldn't attach them. The original photos were bigger than 1MB, but I've created PDF and cropped versions which are well below the 1MB limit, and I still can't attach them - does the site only take certain formats?


Many thanks!!