The police all, without exception, accepted murder-suicide completely - even the relatives. I think the only individuals ahead of the curve were DS Jones and DI Cook.
I think in the case of the relatives it was more that they persuaded themselves of Jeremy's guilt over time and saw things in hindsight. If you look at their statements, you'll notice that the early suspicions are not vocalised to third parties.
Bews claims he was suspicious, but I think that is wisdom after the fact. His concerns about Jeremy arose from the unusual nature of the situation rather than a specific suspicion that all was not as it seemed.
The taking of statements from Jeremy, Colin, et al, was a formality. It was Jeremy's behaviour that drew the suspicion of DS Jones. I think ILB is correct that it was detective instinct and intuition combined with DS Jones' own prejudices that focused his mind on Jeremy. However, Jones was unsure of his own suspicions and went to great lengths to defend Jeremy at first to the relatives, as he was disgusted by their behaviour.
I don't think DI Cook suspected Jeremy as such, but he sensed something was not quite right about it all - again, instinct or intuition. This led him to order more thorough scene of crime work than otherwise would have occurred. But I suspect DI Cook had inner doubts right to the very end, maybe until his death. I see him as a very interesting figure: a sceptic who was perhaps dragged along on a persecutory wave (though sometimes persecutors do catch the right person, and they probably did here).
Initially it was just DS Jones, though as I have said, even he defended Jeremy in front of the relatives, so he can't have been sure of his suspicions for quite some time. What I am saying about DI Cook is that I see him as the 'scientific sceptic'. I think he was intuitively sceptical of the murder-suicide theory, but also sceptical of Jeremy's guilt. Unlike Ainsley and Jones, I don't believe Cook ever came to a firm view.