looking at the source code for your page, it looks like the code that updates the price is:
function updatePriceDisplay() {
var priceInfo = (silent_form[1]).split(":");
document.getElementById("productCode").innerHTML = priceInfo[1];
document.getElementById("productPrice").innerHTML = "£" + priceInfo[0] + " inc. vat";
}
I Would change that to:
function updatePriceDisplay() {
var priceInfo = (silent_form[1]).split(":");
if(priceInfo[1] !="") { document.getElementById("productCode").innerHTML = priceInfo[1]; }
if(priceInfo[0] !="") { document.getElementById("productPrice").innerHTML = "£" + priceInfo[0] + " inc. vat"; }
}
so that the price displayed is only updated if a price is returned by the lookup.