I’m afraid the previous three episodes were really uneven, but Randall & Hopkirk went out on a high note written by Mark Gatiss and Jeremy Dyson as a very cute tribute to The Avengers. It’s “Death at Bargain Prices” crossed with “The House That Jack Built” as Jeff and Jeannie are trapped in an escape-proof department store full of lethal traps. And just to add to the tips of the bowler, they brought along some mannequins that evoke the Autons from Doctor Who and dressed one of them like Steed.... And of course I was smitten. This series manages to play tribute to just about every classic TV series and film ever made, including often focusing on the unreal Britain of The Avengers.
That is particularly apparent in this episode, which is a bit of a tribute to The Town of No Return, with nods to The Wicker Man and endless horror films. This page does a better job of identifying cultural references than I would.
I would however comment that this episode is firmly in the genre of literature portraying villages as not quite what they seem. This can include various murder mysteries and much folklore including folk horror. The fact that the residents are ruthlessly organic and middle class adds to the awfulness. As a town person myself I don't feel frightened of city living in the slightest, but the thought of the country fills me with horror.
I can quite see why this is not a favourite of the fans - if you are inclined to you can see the ending of this episode as a weakness, since it is frankly extraordinary. Mary's behaviour doesn't come across as good, but of course it all ends alright. Personally I think a series finale when one of your leading characters is dead, would have to include the dead antics of some power crazed nutters intent on world domination. And how Avengers is that?