Thanks Kor,
I have done what you said but it is still not working 
Below I have included the script for adding a new set of form fields
var counter = <?PHP if($id != NULL) echo $partscount-1; else echo $partscount = 1;?> ;
function addPart(divName){
var newdiv = document.createElement('div') ;
newdiv.setAttribute('id', 'newpartf') ;
newdiv.innerHTML = "<div id='cbox2'><input type='text' value='1' size='1' name='quantity" + (counter) + "'></div><div id='cbox'><input type='text' size='10' name='manu" + (counter) + "'></div><div id='cbox'><input type='text' size='12' name='part" + (counter) + "'></div><div id='cbox'><input type='text' size='12' name='supplier" + (counter) + "'></div><div id='cbox'><input type='text' size='12' name='type" + (counter) + "'></div><div id='cbox'><input type='text' size='10' name='delivery" + (counter) + "'></div><div id='cbox'><select name='ourcurrency" + (counter) + "'><option vale='pounds'>£</option><option vale='euro'>€</option><option value='dollar'>$</option><option value='yuan'>¥</option></select><input type='text' size='2' name='ourcost" + (counter) + "'></div><div id='cbox'><select name='ourcurrency" + (counter) + "'><option vale='pounds'>£</option><option vale='euro'>€</option><option value='dollar'>$</option><option value='yuan'>¥</option></select><input type='text' size='2' name='total" + (counter) + "'></div><div id='cbox'><strong>£<span id='meh" + (counter) + "'>0.00</strong></span></div> \r" ;
document.getElementById(divName).appendChild(newdiv) ;
counter++ ;
}
var counterx = 0 ;
function calcu() {
var targetmeh = "meh" + counterx ;
var quantity = "quantity" + counterx ;
var costtotal = "costtotal" + counterx ;
var costours = "costours" + counterx ;
document.getElementById[targetmeh].innerHTML = Number(document.f1[quantity].value) * (Number(document.f1.[costours].value) - Number(document.f1[costtotal].value));
counterx++ ;