FWIW - think I've solved it with a separate function and
eval():
Code:
<input type="button" id="butt" value="Go" onClick="whiskaway('<?=$d['ID']?>');" />
<script>
function whiskaway(n) {
var f = "dupDetail.php?id=<?=$d['ID']?>&dt=" + eval("document.getElementById('dupdate<?=$d['ID']?>').value");
parent.location=f;
}
</script>