I’m pretty sure I read in one of the West books that Fred West was a ‘grass’, and did have some kind of relationship with the local police, but that’s not quite the same as the police using his ‘brothel’ and covering up what he did. Mary Bastholm was very early on in his sordid life, and when everything fell to pieces for him it seemed to do so quite quickly, mainly due to the efforts of DC Hazel Savage, apparently, who pushed.
Mind you, Caroline Owens was treated abominably by authorities, apparently.