Oh, this is a tricky, tricky questions. It's a good question but not so easy to answer.
For me, the idea comes from something very small. It can be something I read or something I hear and it lurks away at the back of my mind, just coming to the fore every now and then. I think its a good idea keep to notebook for this purpose. Many of the ideas come to nothing but some are more persistent.
Treated as Murder came from this picture I had in my head of an unmarried sister and brother living in Yorkshire. I knew in my mind that the first World War played a part. It was also clear to me that mental illness would play a part. This came directly from my experience of working as a psychiatric nurse.
The idea of having a second underlying story came from my crime novel reading. I think this can work really well though you can get it wrong and I know I committed the sin of letting a sub-plot take too much prominence in (an earlier) draft of Treated as Murder.
I honestly can't say for sure that I initially intended Edith Horton to become heroine of a series. I think this happened gradually.