I didn’t make the Orfordness trip last night due to a leaky car but on the way home from the garage this morning I cycled up to Orwell Country Park to have a look at the Prickly Lettuce and managed to find 4 small larvae on the seedheads. I last checked about a couple of weeks ago so it would seem that they’re just appearing now.
There’s been a record of the adult moth at Stowmarket this year so good places to look might be in the vicinity of the A14 from Felixstowe to Stowmarket and beyond, although if you see the foodplant anywhere it would be worth checking.
Here’s picture of a similar-sized larva that I took last year. I’ve also seen green forms of the larva.