OWA access over internet on Exchange 2003 cluster

We have Exchange 2003 (SP2) installed on Windows 2003 (SP2) cluster as an Active/Passive. Our exiting standalone Exchange servers have a Front-end server that users connect to webmail via OWA over the internet with that works fine. When we move a mailbox over to the Exchange cluster that user can’t connect to webmail. It loops back to login page. Using OWA internally and entering the URL of the mailbox owners clutered Exchange server directly works i.e http://excluster1/exchange works but https://webmail.mydomain.co.uk doesn’t.

We use SSL and all routing etc. is setup like the existing Exchange servers.

We’ve tried modifying the host headers as specified in MS KB287726 but to no avail

Any help gratefully received