After your reply we decided to take the car back to the Volvo dealership to have the boost pressure sensor replaced as it was now misbehaving more. When we tried to drive it there we could not get it above 7 mph so went home and decided to have it towed to the garage, hoping the RAC would do that for us. The mechanic came, was here for about 20 minutes, found that something had come loose and a flap wasn't working. He put a screw in and now it works perfectly. We now have no faith in 'experts' particularly in Volvo dealerships who had the car for a week and didn't find the problem.