I managed to do this by altering the Custom rules in fiddler:

If you edit the OnBeforeResponse
function thus:
static function OnBeforeResponse(oSession: Session) {
if (oSession.HostnameIs("www.myhost.com")){
oSession["response-trickle-delay"] = "400";
}
if (m_ShowTimestamp){
oSession["ui-customcolumn"] = DateTime.Now.ToString("H:mm:ss.ffff") + " " + oSession["ui-customcolumn"];
}
if (m_ShowTTLB){
oSession["ui-customcolumn"] = oSession.oResponse.iTTLB + "ms " + oSession["ui-customcolumn"];
}
if (m_Hide304s && oSession.responseCode == 304){
oSession["ui-hide"] = "true";
}
}
The important bit being:
if (oSession.HostnameIs("www.myhost.com")){
oSession["response-trickle-delay"] = "400";
}
The 400
value is 400ms per KB downloaded (my web service has a loonnngg timeout)
with help from Performance Testing on telerik web site and FiddlerScript CookBook