Thanks for the reply,Bob, but the problem is that I can't get to the ends of the roller. It's covered by an inset panel on the inside of the caravan door. It's this panel with no visible fixings that I need to remove to be able to tension the roller as you have advised. I've tried a bit of prying but the plastic isn't very flexible and I'm a bit scared of smashing it.
Paul, June 2009