Thats correct you can only add a maximum of 50% to the award Tking into account the £750 new car payment andtopping up the award by 50% it only gives you £2250 leaving you £445 short!
You grants manager is likely to say no but, its worth speaking with them and askng them if you can the £445 ontop of everying else. You could also suggest that your likely to keep the car 5yrs to see if thats makes a difference, whether you are or not is irrelevant.