Although the hit BBC series was set in Birmingham, its creator Stephen Knight said it was mostly shot in Manchester, Leeds and Liverpool, but added “this time I thought it was really important to come home”.
Filming was shot across Birmingham, but also part of Northamptonshire.
Harper said: “There’s a tunnel that we use that forms a very important part.
“It’s very, very beautiful in all sorts of ways, it’s one of those treasures that you sometimes find when you’re filming.”
With a bigger film budget it meant “we could go to all these incredible places and find all these little known gems”.