I've binged and thought it was probably one of the weakest ones.
I think this is likely due to the USP of the murders supposedly being from long ago, and the dead people long since forgotten, hence the name of the series, being the big hook for me!
A murder from not long ago is just another police procedural for me, so it lost something even before it started, if you get me. The previous series was similar, as it crept more away from the distant past.
The asylum seekers/refugees/illegal immigrant storyline was sooooooo weak! It seemed so unbelievable that Asif's partner would take the rap and end up with no legal consequences.
I also agree that as soon as the lecturer confessed it was obvious it had been her daughter. In fact, I had a feeling this is what had happened before that point, but this only confirmed it.
I couldn't have cared less about the Irish subplot, and surely Catholic priest shagging someone and getting them pregnant is a stereotypical trope done to death, and this one added nothing to that.
Sunny and Leanne? God! Just what we didn't need to add to everything else!
The only saving grace was Ram Sidhu who was a reminder of how amazing this series used to be, Series 4 being the last of the great ones.
I really hope if this series is renewed that we go back to looking at very old murders where the victims are really 'Unforgotten'.