Results of interwar Hull elections can be found in:
Sam Davies and Bob Morley County Borough Elections in England and Wales, 1919-1938: A Comparative Analysis Volume 4: Exeter - Hull Ashgate, 2013, 715pp
Hull features as chapter 9, pages 549-639 By-election results are not included; results for postponed polls are not included.
It is at this point that Davies and Morley's work comes to an end. It strikes me as doubtful that the remaining and putative four volumes will ever be published, especially following the death from cancer, in 2016, of Sam Davies. An obituary from The Guardian can be found here, suggesting that the research for the four remaining volumes was completed.