I would put this on the same level as the recently-viewed Mindhunter series. The balance of personal drama of the protagonists and the reality of solving the crimes was excellently balanced, each supporting and feeding off the other. Friel - indeed all of them - was superb and the mystery of her blackouts and what she could or could not remember was a gripping device that did not seem like a cop-out as such devices can often be.
Pacing was excellent and I got a real sense of the drudgery, the painstaking attention to details and the moments of passionate excitement, set against major disappointments that, I suspect, are an accurate representation of the experiences of the police involved in such cases.
I also, particularly at this time, really enjoyed the sense of being back in London with so many iconic buildings on view - at times I was trying to work out exactly where they were shooting based on that.
The second series maintains and builds upon the strengths of the first in a way that is all too rare - I had the sense that the entire two series were always thought of as one coherent dramatic unit.