Passing a pile of books outside a shop I noticed one was a PAN and surprisingly it credited the cover artist but not someone I’d heard of. Having now looked him up the only reference I can find is on “The Illustrated Gallery” website where they say

“Although a popular fantasy artist in the 1990s, almost nothing is known about David Bergen’s career. He was active in the 1970s, illustrating Sphere’s H. G. Wells’ reprints and the cover for SF Digest (1976), as well as books by Arthur C. Clarke and Samuel R. Delaney. Soon after, he could be found contributing covers to DAW Books in the USA. Bergen then seemed to disappear until 1990 when his work began appearing on various Pan fantasy and SF titles as well as the Puffin editions of Ursula Le Guin’s Earthsea series. He continued to produce covers until at least 1997 when his work again disappears from sight”

The cover I found was from 1980 so it looks like he was working for PAN earlier than 1990 but the challenge now is to see if I can contact him and find other examples of his work.
* UPDATE I think I’ve tracked him down to The Netherlands and an email has been sent, watch this space.