Synch problems

I've just checked on my WUS server synchronization and it appears that recent
synchs have been failing. On further investigation (through Event Viewer),
I've noptioced that the lats succes was on 11th May. No synchs since this
date have been succesful.
I was away on hols on 11th May so no changes to the server could have been
made.
I can confirm that my Synch options page is set to use Microsoft Update and
that Microsoft update is contacable (and runs) on the server itself.
The details of the error I'm getting are:

Windows Updates Services encountered an error.
The upstream Windows Updates Services server was not found. Please check
your update source settings on the Synchronization Options page.
WebException: The request failed with HTTP status 404: Not Found.
at
System.Web.Services.Protocols.SoapHttpClientProtoc ol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
at System.Web.Services.Protocols.SoapHttpClientProtoc ol.Invoke(String
methodName, Object[] parameters)
at
Microsoft.WindowsUpdateServices.ServerSyncWebServi ces.ServerSync.ServerSyncProxy.GetAuthConfig()
at
Microsoft.WindowsUpdateServices.ServerSync.ServerS yncLib.InternetGetServerAuthConfig(ServerSyncProxy proxy, WebServiceCommunicationHelper webServiceHelper)
at
Microsoft.WindowsUpdateServices.ServerSync.ServerS yncLib.Authenticate(AuthorizationManager
authorizationManager, Boolean checkExpiration, ServerSyncProxy proxy, Cookie
cookie, WebServiceCommunicationHelper webServiceHelper)
at
Microsoft.WindowsUpdateServices.ServerSync.Catalog SyncAgentCore.InternalGetConfig()
at
Microsoft.WindowsUpdateServices.ServerSync.Catalog SyncAgentCore.CatalogSyncThreadProcessReal(Boolean allowRedirect)

I'd appreciate any advice.
Many thanks