If they weren't specifically test driving it as agreed, and stopping at the bookies is hadly part of what a reasonabke person would consider part of a test drive, then they were taking it without owners consent, a criminal offence, and deserve to feel the cold steel of handcuffs on their oil stained wrists.
Actually arrested a chap once for exactly this, except he used a customers car for a 200 mile jolly to collect some parts. He'd tried to cover his tracks, topped up the fuel and reset the trip meter, but overtaking the cars owner on the motorway ( owner was in his wife's car) kind of gave the game away. An ANPR check did the rest.
Aside from which, how does a 4 hour test drive help with reproducing a starting issue?