function v_number(number){
                    	var digits="0123456789.£";
                    	if (number=="£") {return false;}
                    	if (number.length==0) {return false;}
                    	for (var i=0;i<number.length;i++){if (digits.indexOf(number.substring(i,i+1))==-1 && number.substring(i,i+1)!="."){return false;}}
                    	if (number<0) {return false;}
                    	return true;
                    }
                    
                    function formatCurrency(number){
                    
                      number = Math.round(number*100)/100;
                    
                    	var decimalPos;
                    	var string = number.toString();
                    	var zerostring = "";
                    	
                    	decimalPos = string.indexOf(".");
                    	
                    	if (decimalPos != -1) {
                    	  if (string.charAt(decimalPos+2) == "") {
                      	  zerostring = "0";
                      	}
                    	} else {
                    	  zerostring = ".00";
                    	}
                    	
                    	return("£" + number + zerostring);
                    
                    }
                    
                    function calculateVAT(){
                    
                    	//load values from form into variables
                    
                    	var frm=document.vatcalc;
                    	var a=frm.start_figure.value;
                    
                      for (var i=0; i < frm.include_vat.length; i++) {
                        if (frm.include_vat[i].checked) {
                          var vatinc = frm.include_vat[i].value;
                        }
                      }
                    		
                    	//initialise those variables for calculation
                    	
                    	var net=0;
                    	var vat=0;
                    	var gross=0;
                    	
                    	//validate inputs
                    	
                    	var errorMessage="";
                    	
                    	if (!v_number(a)) {errorMessage+="Starting figure,\n";}
                    	
                    	if (errorMessage.length!=0) {alert("The following entries are incorrect:\n\n"+errorMessage+"\nPlease correct these entries and\ntry again.");return false;}
                    	
                    	if (a.substring(0,1)=="£") {a=a.substring(1);}
                    	
                      //different calculates if the starting figure does or does not include vat
                      
                      if (vatinc == "yes") {
                        gross = a;
                        vat = (a * (7/47));
                        net = (gross - vat);
                      } else if (vatinc == "no") {
                        net = a;
                        vat = ((a/100)*17.5);
                        gross = ((net-1)+vat+1);
                      }
                      
                    	//load text boxes with results
                    	
                    	frm.output_net.value=formatCurrency(net);
                    	frm.output_vat.value=formatCurrency(vat);
                    	frm.output_gross.value=formatCurrency(gross);
                    	
                    	return false;
                    }