Inspector Gently catches up with Michael Clements in a nightclub in London, which is playing Curtis Mayfield's 'Move On Up'. This track wasn't released until September 1970, and then only as an American album track.
When Inspector George Gently visits the record pressing plant in Washington, County Durham (RCA opened a plant there in May 1970), the walls are adorned with gold and platinum discs. These weren't awarded by the BPI until August 1973, three years after the scene is set.
The MP Michael Clements mentions in his big presentation plans for "A bridge over the Humber".
This was set in 1970, but the Humber Bridge had already been approved by the Wilson Government in 1966. In any case, it is close to Hull and very remote from County Durham. No major roads from County Durham go that way, and it could not conceivably bring economic benefit that far away.
At the beginning, the police want to disperse the protest, but the protesters are also their suspects, so they should perhaps have kept them around to identify and search them.