Thanks to the people at How To Mend It who had previously explained how to do this and saved me a lot of money: http://www.howtomendit.com/answers.php?id=74026 [howtomendit.com]
Prise the inner door trim away from the door. I started at the top and then worked around the outside using a couple of knives to do this. You can see on the table where I broke off one of the clips though ;-)
This pathetic little plastic hook is all that's meant to keep the spring in place.
You can see where the hook used to be.
The other end of the spring fits onto to little hook on the latch fitting.
Heat up a pointed metallic instrument. A bradawl or small screwdriver should do. I don't actually know what this is, I found it in Lin's sewing kit.
Use the hot tool to bore a small hole in the plastic casing.
Thread a cable tie through the hole.
Fit the spring onto the cable tie, pull it reasonably tight and trim the end. Ensure that the latch is properly in its holder (It should be, there's no need to take it out). Fix the other end of the spring to the latch. It should feel tight, but the door release mechanism should now work.
Replace the door trim and have a celebratory drink.
Jon you're a bloody hero , many thanks.