This would be Jimmy Yuill's last appearance as DI Doug Kersey due to Yuill contracting meningitis. His absence from the remainder of the series was explained as a transfer to another force on an investigation.
Jack Shepherd refused to continue in the title role when the producers sacked Jimmy Yuill (Det. Insp. Doug Kersey) for "insurance reasons" after he contracted life-threatening meningitis during filming, and then would not reinstate him even though he made a full recovery. Yuill was cut from the last two episodes and the series was canceled after that.
Devil's Gateway Stone Circle is a fictional location. They probably used scenes based around local Standing stones, otherwise known as menhirs, which are more abundant in the Penzance, Bodmin and West Penwith areas of Cornwall or perhaps even The Nine Maidens stone circle in St Columb near Wadebridge. The stones used as menhirs were often carefully chosen, their shapes having meanings as well as being visually striking: pillars, slender triangles or stones with pointed tops.
The band of musicians playing folksongs in the pub was called Scatter The Mud.