Couple of the drills I bought from Argos came with the metal gears not mounted on the pins. I always take them apart to grease the gears properly anyway. I checked the grub screws and they seem fine. I'm thinking it was just a dodgy unit that was going to break anyway. Changing the axle/grub screw part of the casing seems to have fixed it for now. Going to give it another run in the garden later