I was disappointed with the episode and am surprised no one else has really criticized it. The first series was great - funny and very clever, I think I laughed out loud at least once in most episodes, and found the plots well-constructed because at the end everything suddenly made sense and you wondered why you hadn't seen it earlier.
First of all, this new episode had a few lame attempts at humour (e.g. the tapping with the stick) but nothing that made me laugh (like, for example, the "who'd kill a bride on her wedding day?" from series 1).
Secondly, the plot was awful. Well, maybe not awful, but very average, and a definite cheat in terms of crime drama - the only reason the audience didn't know who'd done it until the end was because they weren't given all the information the detectives were (we weren't shown where the tunnel led, or the family tree they found on the computer). There was no great 'aha' moment where it all clicked - instead, we were given the missing information about the tunnel and the family tree, and the solution becomes obvious. Didn't really need Poole to solve it, did it - Mr Bean would have done as good a job.
So, I enjoyed it as very light escapist entertainment, but was disappointed at the lack of strong humour and good plot that characterised the first series, and which in my mind had put it on a par with the best episodes of Jonathan Creek. Instead, I thought the new episode came across as a poor imitation of Creek or Midsummer, and it's shame the viewing figures were so high for such a weak effort, and new viewers will think that's typical (see the bad review in the telegraph for an idea of what someone new to the show thought of the episode).
I note from Wikipedia that the writer and director were different from any used in the last series - I'm hoping there are some episodes this series that use the old hands (Robert Thorogood in particular seems to have written most of S1) and that this was not a typical offering from series 2, cos I've been waiting for it for a long time.