The onscreen caption says the "March 1974" General Election, but the election was held on the last day of February, the 28th.
At 11m 50s the yellow Hillman Hunter seen outside Thorpe's home at the time of his first wife's death on June 28th 1970 carries a J suffix registration, which could not have been issued before August 1970.
At 18:05 and 45:50 there are shots of Parliament with Portcullis House clearly visible behind it. It did not open until 2001.
At 16:50, the registration plate of the caravan ends in the letter 'T', not issued until 1979, some years after its place in the series.
When the carpet salesman takes a call at home, the TV can be heard in the background; an episode of Fawlty Towers (1975). This scene being set in 1975, it's correct that series one of that show would be airing. However, the dialogue we hear - "would you care for a rat?" - comes from the series two episode Basil the Rat (1979), broadcast in 1979.
The aerial shots of Parliament are clearly modern, the huge Portcullis House building is plainly visible. It was completed in 2001, but the series is set thirty years earlier.