Kevin, Rob, Richard, Thanks for taking an interest. I think the gears are different - from the pictures I've found on the web one has 21 teeth and the other has 20 teeth. From my own measurements I think that the angles are different too.
Jim, Richard - I'm obviously too much of a hoarder. I needed some springs for the chuck too - but when I chuck stuff out I save useful bits like springs - so I looked in the tin marked springs and found three - I've no idea what they came from.
Thanks all for taking the trouble to comment.
Russell