When slowing down the transmission feels as if it is jerking in to the next gear.

After checking fluid level is correct, try this, to see if it improves...
into drive,let car accelerate without any gas(do not touch pedal)to max speed (about 30/35mph).stop,then with 1/3rd gas pedal,accelerate to max speed(do not move foot)stop,then foot hard to floor,upto about 60mph,then stop.then only using a max of 1/3rd gas pedal,drive for about 15miles.you have just reset the drivetrain computer setings....the car is fitted with a computer controlled fuzzy logic gearbox,it will try to make you drive as eco as poss,but if used harshly the gearchange can be harsh up and down.....
if driven gently 4th will engage at 43 mph,with a slight clunk.