The 1959 Future Frigate studies and the path beyond Leander

What was the Royal Navy's 1959 Future Frigate design series?

They are recalled as improved-Leander successor studies on the path toward Type 22, although the precise connection is left unverified.

The 1959 Future Frigate studies are tentatively identified as work on what should follow the Leander class. The recollection is that improved-Leander ideas contributed to later variants and eventually to Type 22, but that lineage is not checked during the answer. The dimensional comparison is more definite: ordinary Leanders are given as 41 feet in beam and broad-beam ships as 43 feet. The two-foot increase explains the joke about how modest the supposedly broad hull actually was.