This was a great answer and works for a Qualcast 320 Eclipse lawnmower as well. Remove the top green cover as above. The only difference that I found was that I did not need to take the motor out. If you remove the old drive belt the pinion is fully exposed. Using mole grips, hold the pinion tightly. I then used the "nail" part of a small cable grip, (those that you use to hold cable to walls etc), to knock out the old roll pin. Any small nail would do so long as it is a smaller diameter than the hole on the pinion. Remove the old pinion and replace with the new one. Use the small nail to line up the holes in the pinion and the spindle on the motor. Tap in the new rolled pin until it is flush with the pinion. You only need to fit the new belt now. Start by looping the belt around the pinion and then around the drive wheel connected to the cylinder cutters. If you move the pulley wheel slowly, the belt will slip over the pulley wheel and will lock into place. Make a few turns by hand to ensure that the belt is secure on the pinion and pulley wheel. Test by plugging in the lawnmower, making sure that no small fingers are near the exposed pinion and pulley wheel. If all is well, replace the top cover as above and then replace the side cover.
Tony Wirral, May 2012