A good question and having a quick look at the series 1 gearbox casing I cannot see an obvious route. The instruction book indicates that oil will overflow from worm gear casing to main gearbox. To be safe I fill the main gearbox via the dipstick hole, a slow process, then fill worm gear via the filler. Later series 2 machines and all series 3 machines had a simple pump to pump oil upto the worm gears from the main box, each had its own dipstick.