Apparently, says the garage, it was something to do with the computer sensors within the gearbox. They tried it out on the hoist and when they put it into neutral, the sensors thought it was in reverse. This didn't physically happen on the open road but would have been the cause for the hesitancy. They subsequently ordered in a new gearbox from South Korea and swapped it over. Have had the car back for a week and it is a totally new vehicle - has the instant responsiveness when you demand it. The garage advised that it was only the second time they had discovered a computer glitch within a gearbox that couldn't be fixed except by total replacement.