I didn't realize you were using a select list and not a checkbox. In that case, since your "no" value in the select list has a value of zero the correct code would be:
$UpdateQuery->bindColumn("SpecialOfferOldPrice", "s", "".($_POST["SpecialOffer"]?str_replace(array("£","%",),"",($_POST["SpecialOfferOldPrice"])):"0") ."", "WA_ZERO");