Level Bust - you're quite right. Although 'PWA was operated by Autair it wasn't one of their machines - HP's demonstrator. Probably leased pending delivery of 'PWB/C/D.
It was though the last survivor of the series 100 Heralds and wasderelict at Sothend by 1982/3 but (I think) subsequently restored and now displayed a t Woodley in BEA colours.