Hello Claire,
On the actual drop feed lever there should be a screw, you may need to take the bottom of the machine off to see it, the screw that you can see that moves towards you may be the screw you are looking for, they differ quite a lot on Bernina machine, I am searching for sites at the moment that may give a schematic in case they had changed it.
Ken,
email me if you can, type in my address as you say it not as you read it, this site does not allow email addresses to be shown that is why I have to type it below that way.
drivewithken at blueyonder dot co dot uk, October 2009