The last ten days of April were very cold, with frosts down south. Consequently, Purple Emperor larvae did not commence feeding until the weather warmed up at the start of May. That's late, very late by modern standards. Although they can catch up if May is fine, at this rate the butterfly is unlikely to appear before the start of July - but watch this space.
Guy is absolutely right, larvae can be found on sunny days in May by spotting their silhouettes through the foliage. Also, in early May few other Lepidoptera larvae are feeding on sallow leaves, so homing in on any edge-of-leaf or leaf-removal feeding can be rewarding - if you can take the eyestrain. Do it on calm days only.
Here is Sir Cloudsley Shovell, photographed in Savernake on Sun 8th, he will soon change into the 4th instar -
I also found one which I thought had been predated in late winter. He had only just started to feed and has scarcely grown -