function doRecalc(target, amt, isChecked){
	if(!isChecked){
		amt = -amt;
	}
	targetContent = document.getElementById(target).innerHTML;
	targetContent = targetContent.substr(1);
	targetContent = parseFloat(targetContent);
	targetContent = targetContent + amt;
	targetContent = targetContent.toFixed(2);
	document.getElementById(target).innerHTML = '£' + targetContent;
}