The manual explains how to change the belts, but almost certainly this method only works on a brand new machine. So - don't try to undo the bolt on the end of the motor pulley, nor try to remove the pulleys themselves. The first is difficult, the second impossible when a bit rusted.

Here's what the do. Tip the machine over on its right hand side. Remove the cutter deck by taking out the split pins. Put aside along with its belt.

Now undo the pulley keeps - the metal rods - on the motor drive shaft. They are held on by gripper nuts at the top and come off easily. They are what was stopping the belts coming off.

Next find the silver coloured knee shape at the lower right, which is part of the cutter brake mechanism. Unscrew the bolt and remove. Now undo the bolt holding the pulley which is just up and to it's left and part of the same system. Pull the pulley forward enough to let the belt clear the keep. Take the belt off. Manoeuvre the other belt past a large spring to take that one off. Now both belts are off and can be replaced. Re-assemble in reverse order.

When all is back together, slacken off the adjusters on the drive belt brake - each end of a long Bowden cable. This is best done with the machine over on its left side.

Done! Took me ages to work that out.