Thanks Sunny for the interesting ID suggestion of E. ada iona.
According to C&P4, E. ada iona has only two cell spots rather than three.
However my poorly taken shots of another two specimens (all very skittish, even in the cold wet weather) did reveal only two obvious cell spots, with the lowest one possibly hidden by the hindwing.

My impression of the butts is that they are about the same size as the other common grass yellows, and not the small size suggested in C&P4.

Horace