Ruth Evershed's love interest in this episode is played by Nicola Walker's real life husband, Barnaby Kay.
The scene of the Mozart scratch performance is meant to be in St Martin in the Field. It's actually filmed in the crypt of St Marylebone, Paddington.
When Adam phones Zoe & Danny to check their current location, Danny states that they are 57° North & 2° East. That would mean that rather than sailing across the North sea, the ferry has sailed almost due north, as Harwich is 1.26° East to start with. The coordinates that Danny gives would mean that they are just 1.6° South of Kristiansand, but 6° West of it. If the ferry was in that location it would have to perform an almost right turn to get to its destination. Even taking into account that it would have to stick to authorised shipping lanes, it is wildly off course.