Gaffes

When David and Alan are reunited, Alan swears an oath to being innocent. In a brief close-up his beard changes.

In the 18th Century, the letter 'S' looked like a long lowercase 'F', but the signage does not reflect this.

While hurriedly climbing to the top of the tower, David could not conceivably have time to grab the wooden beam overhead in the split second while the last step beneath him falls to the ground. The incident in the novel seems more plausible, in which David slowly feels his way through the darkness to the top, discovering that the steps end.