Beginner, built Ender 3, can't get bed level

[Gareth 633]

Hi, I'm a total beginner, I built the Ender 3 3 days ago but can't get the bed to level at all.
My problem is that after auto home, the nozzle is about 8mm above the bed. When I try to raise the bed, the knobs actually fall off with the nozzle still a long way from the bed. I've tried fiddling with the Z axis end stop but it's as low as I can get it.
Any thoughts please?