Other mistake: Kenneth Cope had to wear a toupee for his part. He did so, only it was initially on the wrong way, though no one pointed it out or noticed. If it ain't broke...
A Sentimental Journey - S1-E19
Revealing mistake: Dialogue in the episode specifically mentions the A2 and A282 roads near Dartford, Kent - the actual roads used were the A1 Barnet by-pass, Galley Lane and Well End Road. The error is revealed by the sight of a road sign that appears just as the Austin Princess Limousine turns on the the main road - the sign shows that the car is not turning on the the A2/A282 near Dartford, but the A1 at Barnet some 40 miles away.
Trivia: Mike Pratt (playing Jeff Randall) really was laid up in a hospital bed with two broken legs. Mike Pratt was attempting to climb a drainpipe in a slightly inebriated state during his 38th birthday celebrations. He fell 20 feet into the basement area. He'd forgotten his keys, and saw the stand-in doing it - successfully - first. Mike being Mike, he thought, "I can do that" and had a go. Jeff's 'in character' excuse in the show was that he'd fallen from a balcony in a chase - a little more dignified than the truth.