Mountfield self-drive SP470 lawnmower?

Hi, I have a mountfield petrol SP470 lawnmower selfdrive and need to fit a gearbox tensioner spring? I have fitted new drive belt case, new drive cable but can't find where to fit the new tensioner spring? I cant find a hole on gearbox to sit the spring, I can't find a hole in the case to sit the spring? The area was fu of grass and when I cleared the grass the spring dropped out? Sp I have no idea how it fits? A diagram of its position I can't find anywhere, I bought a new spring too as the old one was very old, rusty and about to break? Can you help me? Many thanks regards Graham.