Ray Bradbury wrote the original Something Wicked script, but the film's director Jack Clayton commissioned a re-write from Mortimer - without telling Ray. This caused a serious rift in their working relationship. Ray and Jack came to the film as friends, but departed as strangers.
[Update for clarity:
Having read Bradbury's screenplay drafts as well as the Mortimer/Clayton version which was filmed, it's clear to me that Bradbury needed no re-writing. What underpinned these shenanigans was a difference of philosophy between Bradbury and Clayton. Bradbury rightly believed that fantasy stories need plotting that carefully builds, and this can include partial repeating of, or reminders of, events that have gone before. Clayton on the other hand believed that it was wrong to have repetition in a script, and that scares and suspense required constant novelty. He repeatedly expressed this as "a mouse doesn't come out of the same hole twice".]
Ironically, two years earlier Mortimer was himself a victim of an uncredited rewrite, when his scripts for the award-winning Brideshead Revisited TV series were scrapped by director Charles Sturridge. Mortimer retained the sole script credit for Brideshead (and, presumably, entitlement to any royalties), just as Bradbury retained the sole credit for Something Wicked.
In this BBC Archive clip, John Mortimer talks about his father and Rumpole: https://twitter.com/i/status/1649331741155262464