Alas, I did not yet discover all of the names of the Illustrators from the original hardcover and paperbacks who designed the jackets. This is what I found so far:

1. One love is enough - jacket Illustrator: Pino dell'Orco (Hardcover #1)
2. Catherine - jacket Illustrator: Jon Davis (Hardcover #2)
3. Belle Catherine - jacket Illustrator ?
4. Catherine and Arnaud - jacket Illustrator: David Grove (Hardcover #4)
5. Catherine and a time for love - jacket Illustrator ?
6. A Snare for Catherine - jacket Illustrator ?
7. Catherine: The Lady of Montsalvy - Ilustrator: Martin Baines paperback #7) NEW

The same goes for the English and Canadian paperbacks

1. One love is enough - jacket Illustrator?
2. Catherine - jacket Illustrator: Gino d'Achille (for Pan books GB)
3. Belle Catherine - jacket Illustrator : Hans Heinrich Helweg for the Canadian edition
4. Catherine and Arnaud - jacket Illustrator?
5. Catherine and a time for love - jacket Illustrator ?
6. A Snare for Catherine - jacket Illustrator ?

15 February 2020
I am really excited to announce that I finally know the name of the Artists for six of the Pan books covers for the CATHERINE Series. It was the great talented Hans Heinrich Helweg from Denmark and acclaimed Italian Artist Gino d'Achille. A big thank you to Tim Kitchen from www.tikit.net for this splendid news. Hans Helweg was a great Artist, I invite you to click on the link above.

June 2021
... and I am delighted that I have had contact with Mrs. Simona d'Achille who was so kind to confirm to me that indeed her late father the acclaimed Italian Artist Gino D'Achille had illustrated three of the 'Catherine' Pan books we see here below. They are:

ONE LOVE IS ENOUGH - CATHERINE - CATHERINE A TIME FOR LOVE
 

Simona d'Achille has confinded in me that  her father had at times asked her mother and his children to pose for a painting. In this particular case it was for 'Catherine and a time for love'. For this reason the fifth of the Catherine Pan books cover is her favourite and close to her heart. I admit that I am now looking at that particular cover with a tender smile. It is now his daughter who manages his Archive and Studio Editions. link to https://www.ginodachille.com/ website

For the American Book Covers see → here

Evening Standard 13 August 1964

Only original English article so far discovered  by this webmaster about ONE LOVE IS ENOUGH before the Novel was available in England at the bookstores...!

We learn from the small article on the Collage below, how many attempts the Artist tried out until they believed to have found the perfect image for 'Catherine.' Yet the Magazine 'Tatler' did not mention the name of the Artist...! I do not know this for sure, but in the Sixties and even later they did not think this necessary!

Yet in this particular book, Heinmann Ltd. printed in the inside of the dust jacket the name of the Illustrator. His name 'Pino Dell'Orco, a young  and very talented Italian Artist, invited to come to London by 'Renato Fratini', another great Artist from Italy. Later 'Dell'Orco' would get famous for his illustration for some of the 'Spy Thrillers' written by 'Desmond Bagley'! I owe this news to my friend 'Philip Eastman' from the official Desmond Bagley website., researcher and author of the E-book: Desmond Bagley Writer : An Enguiry into a Novelist

16 February 2020
When I saw on the inside of the original dust jacket the name of the Artist DELL'ORCO. I could not resist and did some research about the Artist on Google. I was lucky and found The Bagley Brief website (about the English Author Desmond Bagley (1923 - 1983) where the webmaster, Researcher and Author informs us that Pino Dell'Orco (born ? - 2013) was a renowned book cover Artist. Thanks to Philip Eastwood we are able to learn and admirer other Works by Pino Dell'Orco. I am very much honoured that the Book cover of ONE LOVE IS ENOUGH is now also immortalised there thanks to Phil.

Juliette Benzoni 1964 in London at a Cocktail party given in her honour by Heinemann Ltd, accompanied by the Press agent Gérald Gauthier. of Opéra Mundi.. on the second image above, we see BBC reporter 'Leigh Crutchley' interviewing  Benzoni.