The headrests in Gwenda's car disappear then reappear between shots as she is leaving the railway station.
When Brian Stimpson and Laura stop the Porsche the driver states that the car was new and had only "12 miles on the clock" After Laura steals the Porsche we see a close up of the tachometer the clock clearly reads that the car has 754 miles "on the clock."
When Stimpson is informed the train for Norwich is on the adjacent platform, he could not have been on the Plymouth train, as that would leave from London Paddington, not London Liverpool Street which is where Norwich trains depart from.
(at around 1h 23 mins) By the end of the movie when Stimpson is delivering his speech and the left arm coat sleeve falls down, we can see clearly that is John Cleese himself that pulls something with his right hand making the sleeve fall.