Hi Ben - exactly as you describe. It has been a long time since I’ve done this on an 851, but I think it’s the same as the 888. Remove the filler cap via three screws, then the fixture that it fits into is secured by three grub screws. Once those grub screws are loosened, the assembly can be removed and the hole is big enough to get your hand in and to access the fuel pump and filter.
You may need to replace the large viton o-ring on reassembly (part 15 in the diagram)
