Matvyn Wright was a long-time collaborator with Maria Bird, producing dozens of drawings for books starring Andy Pandy. I believe Wright was also the original artist for the Andy Pandy comic strip in Robin, although I've only seen a small selection of his work so I have little to compare. He was certainly credited with producing artwork for Robin Annual in the mid-1950s.
There is very little on Matvyn Wright on the net. The War Museum site reveals that he was born in 1910 and worked for the Auxiliary Fire Service during the Second World War. He "witnessed with his colleagues the extraordinary strange silhouettes and falling forms of the Thames Blitz." A painting produced by Wright in 1941 and held by the War Museum captures "the fear of waiting and destruction. The bomb almost appears to be held up by the crane-like structures of the barge. In reality, this is a fragile snapshot before the inevitable explosion, with the fire crew poised to respond."
I can add a small amount of biography to this. William Matvyn Wright was born on 4 August 1910 and studied at the Royal College of Arms.
Around 1950, he appeared on TV fronting the show Round the Galleries (1950). He was on the animation crew for Halas & Batchelor's classic Animal Farm (1954). In later years he was a supporter of the Aberdyfi Art Society.
He was married to Kaye Wright and died in Camden, London, on 17 November 1983.
As well as Andy Pandy, I think Matvyn Wright was also responsible for the weekly adventures of Bill and Ben, also published in Robin.
He was certainly a prolific illustrator, as will have been seen in the bibliography of Maria Bird. Since that listing still needs some work, I've listed only a number of titles not previously listed that Matvyn Wright illustrated.
Watch With Mother. Pictures, stories, puzzles and games based on Watch with Mother television programmes, ed. Freda Lingstrom; illus. with others. London, Publicity Products, 1955.
Five Listen With Mother Tales about Charles by Ruth Ainsworth. London, Adprint, 1957.
Kami the Sherpa by Showell Styles. Leicester, Brockhampton Press, 1957.