Since deleting several suspicious items which Sophos Antivirus found I managed to stop Outlook Web Access from working. When I have looked into this further it appeats from an error that there are more serious issues.
When trying to start IIS the following error appears in system event log:
Event Type: Error
Event Source: Service Control Manager
Event Category: None
Event ID: 7003
The World Wide Web Publishing Service service depends on the following nonexistent service: HTTPFilter
I can not see how to resolve this and am not quite desperate to get OWA working again.