Just finished Ben Elton's Time after Time.
Historically I have enjoyed the ideas in his books sufficiently to overlook the shortcomings in the writing; his last - two brothers - I thought was really very good.
Time after time, unfortunately, is a mediocre SF idea, featuring Newton as appears obligatory these days (cf Rivers of London) with a plot so thin it could form the basis for a new diet. The characters are badly formed, two dimensional caricatures (ok,not unusual for BE), there's only one plot twist which isn't transparent, and that's because it's a bit "so what?". Some interesting background which may be factual around the detail of the assassination of Archduke Ferdinand, but that can only hold attention for a short while. All in all, save your money and time.