I enjoy Garrow's Law too. The attention to costume detail is fine. But the 18th century "periodspeak" is flawed--a valid turn of phrase of the time is often followed by vocabulary as spoken by a youngish person of 2011. As for the background political history which is included, that is factually wrong to a high degree.
Lord Melville, a baddy who Garrow wants to see impeached because of his interference with justice in the English courts was
a)not called Melville till 1802--at the time of most episodes he was Henry Dundas
b)was the government's chief law officer in Scotland, not England
c)had a well-known Scots accent.
Other misrepresented characters include Charles Fox.
The criminal law and punishment part of "Garrow's Law" is spot-on, and after all that's the focus of the plots. But there was no need to get the supporting aspects so wrong.