The idea is good. Treachery abounds in restoration London. Without giving the story away too much, the hero has a few days to reveal and thwart a dastardly plot. However...
There are too many coincidences, characters and suspensions of disbelief required to make it a worthwhile read. For example: Our hero seems to gain most of his information by hiding behind convenient curtains, pillars and other obstacles whilst the bad guys hold revealing conversations a few inches away. At some point you wonder if all the residents of London are blind or just stupid when it comes to careless talk.
Some plot twists are not explained, some red herrings are left hanging, what was wrong with the Thames tides, for example. So much so that an Epilogue is required to recap the conclusions and explain the bits left hanging.
All in all, not a good read.