Writing a Spin Off Series

In all my interviews and posts about writing a spin off series, I never considered the possibility of confusing my two crime series. Geraldine Steel is a DI on the Met in London, while her former sergeant, Ian Peterson, is now a DI working in York. Clear enough?
Today I sat down to type the next chapter of the third book in the Ian Peterson series.
Let me give you the context. I'll try not to confuse you. I completed the seventh Geraldine Steel book, Killer Plan, before starting on the third Ian Peterson. That manuscript was interrupted for about a month while I was away at a literary festival in France, and then dealing with the edits for Killer Plan.
It's probably the case for most authors, that when I'm writing a book the narrative lodges inside my head like a giant coiled up worm that has to be unwound and released. Once a story is inside my head, it occupies my thoughts until I've written it out. Half way through the third Ian Peterson story, I had to abandon it temporarily and it went out of my head. My internal 'reality' changed.
Are you still with me?
So today I returned to the third book in the Ian Peterson series. The chapter I was working on opened with the local profiler. Chancing to check on another character in my 'Character Notes' document, I realised that I had introduced the wrong profiler into the Ian Peterson story! Instead of Ian's sympathetic clever male profiler from York, the rather irritating female London profiler from the Geraldine series was there discussing Ian's case with him in York.
Of course I would have realised, and my Editor would certainly have noticed, so there's no way that could have slipped through the net to publication. Even so... I'm going to have to watch out! Writing a spin off series has more potential pitfalls than I realised...