I haven’t read the books, so I feel I can judge the film on its own merits without comparing it to the books.
Apart from the obvious mis-casting of Pierce Brosnan and Tom Ellis (neither particularly believable as Essex hardmen/ ex-union rep), I thought the casting was good and Helen Mirren put in a strong and believable performance as an retired secret agent.
In general most of the film was an enjoyable watch, with some light humour although it did feel a bit too unrealistic in parts to come across as a believable crime caper. One example that comes to mind was when one of the main suspects drops down dead in front of the crowd and Joyce, an ex-nurse, proclaims him as dead without anyone even attempting CPR, another example was that the local police detective had no idea who the crime king-pin in the photo was, and had to rely on the rookie to identify them. Bogden randomly digging up bones in the cemetery made no sense at all, and his role in the conclusion also felt unsatisfying and rushed.
The part which really spoilt my overall enjoyment of the film was the conclusion of the historic murder. I had high hopes that this would have had more connection to the main plot or been revisited more frequently during the film which wasn’t the case. The way that Elizabeth came to her conclusions was not based on any solid evidence at all, and how they then approached the suspect was cruel and insensitive. They were willing to turn a blind eye to the organised crime boss keeping his freedom but they felt it necessary to report their long-time friend to the police in the knowledge of what the likely outcome would be.