function calculateCost(gasform) {
  var distance = gasform.journeyDistance.value;
  var costPerLitrePetrol = 0.94; // current price per litre petrol
  var costPerLitreDiesel = 1.04; // current price per litre diesel 
  var costPerGallonPetrol = costPerLitrePetrol*4.54; // convert price per litre to price per UK gallon
  var costPerGallonDiesel = costPerLitreDiesel*4.54; // convert price per litre to price per UK gallon
  var mpgPetrol = 35; // average mpg petrol for UK car
  var mpgDiesel = 50; //average mpg diesel for UK car
  
  
  document.gasform.JouneyCostPetrol.value = "£" + Math.round(distance/mpgPetrol * costPerGallonPetrol);
   document.gasform.JouneyCostDiesel.value = "£" + Math.round(distance/mpgDiesel * costPerGallonDiesel);
}