Complex sentence: Hugh Greville Spencely (1900-1983) was a British architect, mostly working in partnership with Sir Anthony Minoprio (1900-1988), who were friends since both were schoolboys at Harrow School.
Split into simpler sentences: Hugh Greville Spencely (1900-1983) was a British architect. He mostly worked in partnership with Sir Anthony Minoprio (1900-1988), the two having been friends since they were schoolboys at Harrow School.