I was being very stupid indeed. The jquery ajax method defaults the type to a GET as soon as I changed it to POST it worked fine
function registerApp(){
var xml='<QUERIES><QUERY CMD="REGISTER"><CLIENT>xxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx</CLIENT></QUERY></QUERIES>';
xmlDoc=$.parseXML(xml);
$.ajax({
type: "POST",
url:"https://cxxxxxxxx.web.cddbp.net/webapi/xml/1.0/",
processData:false,
data:xmlDoc
});
}